Lymphocyte Differentiation and T-cell Structure
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Questions and Answers

What is the main function of the CD3 protein in T cells?

  • To serve as a coreceptor for the TCR
  • To transmit TCR recognition signals inside the cell (correct)
  • To initiate apoptosis in T cells
  • To stabilize the TCR structure

    • What occurs during the positive selection of T-cell precursors in the thymus?

  • They rearrange their TCR genes to create unique receptors (correct)
  • They lose their CD4 and CD8 expression
  • They migrate directly into the circulation
  • They undergo apoptosis due to strong self-binding

    • Which of the following accurately describes the T cell receptor (TCR) structure?

  • It has one alpha and one beta chain fixed to the cell membrane (correct)
  • It is exclusively located inside the cell without any external domain
  • It consists of three chains: alpha, beta, and gamma
  • It is composed of four chains connected by CD3

    • What role do CD4 and CD8 proteins play in T cells?

    <p>They enhance TCR signal transmission through cytoplasmic tyrosine kinases</p> Signup and view all the answers

    In negative selection of T cells, which outcome is expected for cells that strongly bind to self-antigens?

    <p>They undergo apoptosis and are eliminated</p> Signup and view all the answers

    What is the primary characteristic of naïve T cells after leaving the thymus?

    <p>They are unable to recognize any antigen until activation</p> Signup and view all the answers

    What best describes the hypervariable region of a TCR?

    <p>It is responsible for binding to peptide–MHC complexes</p> Signup and view all the answers

    What occurs after T-cell precursors successfully bind to MHC during positive selection?

    <p>They have the potential to undergo negative selection next</p> Signup and view all the answers

    What is the primary function of CD8 positive T cells?

    <p>Recognize and destroy cells with foreign proteins</p> Signup and view all the answers

    Which of the following categories does NOT belong to CD4 positive T cells?

    <p>Cytotoxic T lymphocytes (CTLs)</p> Signup and view all the answers

    Which types of CD4 positive T cells are involved in coordinating immune responses in inflamed tissues?

    <p>Th-1 cells</p> Signup and view all the answers

    What is the role of B lymphocytes in the immune system?

    <p>Producing antibodies and forming memory B cells</p> Signup and view all the answers

    Where do B-cell precursors primarily arise from during early development?

    <p>Fetal liver</p> Signup and view all the answers

    What is the role of Bruton’s tyrosine kinase (BTK) in the maturation of B cells?

    <p>It detects functional immunoglobulin and signals for differentiation.</p> Signup and view all the answers

    What is the significance of TCR-peptide-MHC recognition for T cells?

    <p>It is necessary for initial priming and effector functions</p> Signup and view all the answers

    Which type of T cell is specifically tasked with suppressing inflammation?

    <p>Regulatory T cells (Treg)</p> Signup and view all the answers

    Which of the following statements about antibodies is correct?

    <p>Antibodies are glycoproteins made of both light and heavy chains.</p> Signup and view all the answers

    What happens to circulating B cells if they are not activated through their antigen receptors?

    <p>They undergo apoptosis within days or weeks.</p> Signup and view all the answers

    Which surface molecule is indicative of a mature CD4 positive T cell?

    <p>CD4</p> Signup and view all the answers

    Which sections of the antibody molecule directly participate in antigen binding?

    <p>The variable regions of the light and heavy chains.</p> Signup and view all the answers

    What is a consequence of a mutation in the gene encoding BTK?

    <p>Development of X-linked agammaglobulinemia.</p> Signup and view all the answers

    What characterizes the antigen binding fragment (Fab) of an antibody?

    <p>It includes variable regions and binds to the antigen.</p> Signup and view all the answers

    During which phase of B cell maturation does recombination of immunoglobulin genes occur?

    <p>Exclusively during the antigen-independent phase.</p> Signup and view all the answers

    What is the approximate molecular weight of light polypeptide chains in antibodies?

    <p>25,000.</p> Signup and view all the answers

    What is the main requirement for B cells to exit the bone marrow?

    <p>They must not strongly bind self-antigens.</p> Signup and view all the answers

    Which function is NOT associated with T cells?

    <p>Production of antibodies.</p> Signup and view all the answers

    Which characteristic distinguishes natural killer (NK) cells from B and T lymphocytes?

    <p>They lack T-cell receptor and CD3 proteins.</p> Signup and view all the answers

    What role do plasma cells play in the immune response?

    <p>They secrete antibodies.</p> Signup and view all the answers

    What do NK cells use to kill infected or cancerous cells?

    <p>Perforin and granzyme.</p> Signup and view all the answers

    What is the primary function of the Fc fragment of an antibody?

    <p>It attaches the antibody to host cells.</p> Signup and view all the answers

    Which statement is true regarding plasma cells?

    <p>Plasma cells release thousands of antibodies per second.</p> Signup and view all the answers

    During which process do B cells that generate a self-reactive BCR get eliminated?

    <p>Negative selection</p> Signup and view all the answers

    What occurs when mature memory cells are exposed to their specific antigen?

    <p>They immediately differentiate into plasma cells.</p> Signup and view all the answers

    What is the role of immunoglobulin gene rearrangement in B cell maturation?

    <p>It allows B cells to undergo positive selection.</p> Signup and view all the answers

    What distinguishes the B cell receptor (BCR) from T cell receptors?

    <p>BCRs can bind a wider variety of antigens.</p> Signup and view all the answers

    What is a significant characteristic of memory cells compared to plasma cells?

    <p>Memory cells do not produce antibodies unless activated.</p> Signup and view all the answers

    Which antibody types are linked by the constant region (Fc) to form larger multimers?

    <p>IgA and IgM</p> Signup and view all the answers

    T lymphocytes develop primarily in the bone marrow.

    <p>False</p> Signup and view all the answers

    Naïve lymphocytes have encountered foreign antigens.

    <p>False</p> Signup and view all the answers

    B lymphocytes are responsible for cellular immunity.

    <p>False</p> Signup and view all the answers

    Memory cells arise from activated B and T lymphocytes.

    <p>True</p> Signup and view all the answers

    T-cell precursors are produced from common lymphoid progenitors.

    <p>True</p> Signup and view all the answers

    The major function of NK cells is to produce antibodies.

    <p>False</p> Signup and view all the answers

    The process of clonal selection is involved in T and B cell maturation.

    <p>True</p> Signup and view all the answers

    Plasma cells are a type of naïve lymphocytes.

    <p>False</p> Signup and view all the answers

    CD4 positive T cells can be categorized into regulatory T cells, which enhance inflammation.

    <p>False</p> Signup and view all the answers

    Cytotoxic T lymphocytes are identified by CD4 markers.

    <p>False</p> Signup and view all the answers

    B cell precursors primarily migrate to the bone marrow after initially arising in the fetal liver.

    <p>True</p> Signup and view all the answers

    T cells do not require MHC recognition for their initial priming.

    <p>False</p> Signup and view all the answers

    Follicular helper T cells assist B cells within the B-cell follicle of the lymphoid organ.

    <p>True</p> Signup and view all the answers

    The primary function of CD8 positive T cells is to produce antibodies.

    <p>False</p> Signup and view all the answers

    Naïve T cells undergo thymic selection to become mature T cells.

    <p>True</p> Signup and view all the answers

    Memory B cells serve a single purpose to clear infections.

    <p>False</p> Signup and view all the answers

    T cell receptors are composed of three chains instead of two.

    <p>False</p> Signup and view all the answers

    During negative selection, T cells that bind too weakly to self-antigens are eliminated.

    <p>False</p> Signup and view all the answers

    CD3 proteins are essential for TCR signal transmission from the inside to the surface of the cell.

    <p>False</p> Signup and view all the answers

    Double-negative T-cell precursors in the thymus can directly recognize antigens.

    <p>False</p> Signup and view all the answers

    The cytoplasmic tail of the T cell receptor is responsible for attaching the receptor to the cell membrane.

    <p>False</p> Signup and view all the answers

    T cells exit the thymus as mature naïve T cells after undergoing positive selection.

    <p>True</p> Signup and view all the answers

    Coreceptors CD4 and CD8 assist TCR binding by interacting with polymorphic regions of MHC.

    <p>False</p> Signup and view all the answers

    In the thymus, T-cell precursors that do not bind to MHC are chosen to survive.

    <p>False</p> Signup and view all the answers

    Bruton's tyrosine kinase (BTK) is crucial for the differentiation of B cells into plasma cells.

    <p>False</p> Signup and view all the answers

    Once produced, mature B cells do not require activation to survive in circulation.

    <p>False</p> Signup and view all the answers

    Mature B cells express immunoglobulin (Ig) as an IgD molecule on their surface.

    <p>False</p> Signup and view all the answers

    The antigen binding fragment (Fab) of an antibody is comprised of both the variable and constant regions of the light chain only.

    <p>False</p> Signup and view all the answers

    The molecular weight of heavy polypeptide chains in antibodies can exceed 70,000.

    <p>True</p> Signup and view all the answers

    The Y shape of the simplest antibody molecule consists of three polypeptide chains: two identical heavy chains and one light chain.

    <p>False</p> Signup and view all the answers

    A mutation affecting the gene encoding BTK leads to a condition where antibodies can still be produced by B cells.

    <p>False</p> Signup and view all the answers

    Light polypeptide chains in antibodies have a higher molecular weight than heavy polypeptide chains.

    <p>False</p> Signup and view all the answers

    The Fc fragment of an antibody is crucial for binding to antigens.

    <p>False</p> Signup and view all the answers

    Memory cells remain functionally inactive until they encounter an antigen again.

    <p>True</p> Signup and view all the answers

    A B-cell precursor that cannot generate a functional B cell receptor (BCR) survives the selection process.

    <p>False</p> Signup and view all the answers

    IgM and IgA antibodies can combine to form larger “dimers” via the Fc region.

    <p>False</p> Signup and view all the answers

    Plasma cells can release up to 2,000 antibodies per day once matured.

    <p>False</p> Signup and view all the answers

    The negative selection process in B cells occurs exclusively in peripheral tissues.

    <p>False</p> Signup and view all the answers

    BCRs are capable of binding to proteins, lipids, carbohydrates, or nucleic acids.

    <p>True</p> Signup and view all the answers

    All pre-B cells that generate surface IgM advance through positive selection.

    <p>True</p> Signup and view all the answers

    How do T cell receptors (TCRs) achieve specificity for their antigens?

    <p>TCRs achieve specificity through their unique variable regions that bind to specific peptide-MHC complexes.</p> Signup and view all the answers

    What triggers the transition of naïve T cells into either effector or memory cells?

    <p>The transition is triggered by antigen recognition in combination with TCR signaling.</p> Signup and view all the answers

    Why is negative selection important for T cells during thymic development?

    <p>Negative selection prevents T cells that strongly bind to self-antigens from maturing, reducing the risk of autoimmune diseases.</p> Signup and view all the answers

    Explain the structural significance of the CD3ζ protein in T cells.

    <p>CD3ζ is crucial for transmitting signals from the TCR to the interior of the T cell, enabling an effective immune response.</p> Signup and view all the answers

    What role do the cytoplasmic domains of CD4 and CD8 play in T cell activation?

    <p>The cytoplasmic domains amplify the TCR signaling by recruiting tyrosine kinases important for T cell activation.</p> Signup and view all the answers

    Describe how double-positive T-cell precursors are selected in the thymus.

    <p>Double-positive precursors must successfully bind to MHC presenting self-peptides, allowing survival and maturation.</p> Signup and view all the answers

    What distinguishes the mature naïve T cells that exit the thymus?

    <p>Mature naïve T cells are characterized by the ability to recognize antigens without responding to self-peptides.</p> Signup and view all the answers

    How do the variable regions of TCRs contribute to immune specificity?

    <p>The variable regions possess hypervariable segments that adapt to bind distinct peptide-MHC combinations, ensuring diversity in immune recognition.</p> Signup and view all the answers

    What role does Bruton’s tyrosine kinase (BTK) play in the maturation of B cells?

    <p>BTK detects the presence of functional immunoglobulin on the cell surface and signals pre-B cells to continue dividing and differentiating.</p> Signup and view all the answers

    How do mature B cells with functional antigen receptors interact with antigens during the antigen-dependent phase?

    <p>Mature B cells bind antigens through their B cell receptors, initiating the immune response.</p> Signup and view all the answers

    Describe the structural characteristics of the simplest antibody molecule.

    <p>The simplest antibody molecule has a Y shape, consisting of two identical heavy chains and two identical light chains.</p> Signup and view all the answers

    What is the significance of hypervariable regions in antibody molecules?

    <p>Hypervariable regions are responsible for the specificity of antigen binding and are crucial for the diversity of antibodies.</p> Signup and view all the answers

    What is the effect of circulating B cells not being activated through their antigen receptors?

    <p>If not activated, circulating B cells have a short life span, typically lasting only days or weeks.</p> Signup and view all the answers

    Explain the composition of the antigen binding fragment (Fab) of an antibody.

    <p>The Fab is composed of the variable regions of the light and heavy chains, along with the constant region of the light chain and part of the heavy chain.</p> Signup and view all the answers

    What characterizes the B cell receptor (BCR) of a mature B cell?

    <p>The BCR of a mature B cell is primarily an IgM molecule that includes a tethering region on its heavy chain.</p> Signup and view all the answers

    What must occur for pre-B cells to differentiate into B cells?

    <p>A functional immunoglobulin must be present on the surface of pre-B cells.</p> Signup and view all the answers

    What distinguishes T lymphocytes from B lymphocytes in terms of their development locations?

    <p>T lymphocytes develop in the thymus, while B lymphocytes mature in the bone marrow.</p> Signup and view all the answers

    Define the term 'naïve lymphocyte' and explain its significance in the immune response.

    <p>Naïve lymphocytes are those that have not encountered a specific antigen, making them essential for initiating adaptive immune responses.</p> Signup and view all the answers

    What is the primary function of B lymphocytes in the adaptive immune system?

    <p>B lymphocytes are responsible for producing antibodies that target foreign invaders such as bacteria and viruses.</p> Signup and view all the answers

    Explain the concept of clonal selection in the context of T and B cell activation.

    <p>Clonal selection refers to the process where specific lymphocytes are activated and proliferate in response to their specific antigen.</p> Signup and view all the answers

    What are memory cells and how do they differ from naïve lymphocytes?

    <p>Memory cells are specialized lymphocytes formed after an immune response that can quickly respond to subsequent exposures to the same antigen, unlike naïve lymphocytes.</p> Signup and view all the answers

    Identify two major subclasses of T lymphocytes and their primary functions.

    <p>CD4 positive T cells help coordinate immune responses, while CD8 positive T cells directly kill infected or cancerous cells.</p> Signup and view all the answers

    What is the role of the thymus in the development of T lymphocytes?

    <p>The thymus is where T cell precursors mature and undergo selection processes to ensure self-tolerance and specificity.</p> Signup and view all the answers

    How do natural killer (NK) cells differ in function from T and B lymphocytes?

    <p>NK cells are part of the innate immune system and function primarily to kill infected or tumor cells without requiring prior sensitization to antigens.</p> Signup and view all the answers

    What are the three main categories of CD4 positive T cells and their primary functions?

    <p>The three main categories are effector/helper T cells (Teff or Th), follicular helper T cells (Tfh), and regulatory T cells (Treg), with roles in coordinating immune responses, assisting B cells, and suppressing inflammation respectively.</p> Signup and view all the answers

    How do cytotoxic T lymphocytes recognize and eliminate infected cells?

    <p>Cytotoxic T lymphocytes recognize infected cells by binding to foreign proteins presented on MHC molecules via their T cell receptors (TCR), and they induce apoptosis in these infected cells.</p> Signup and view all the answers

    Describe the origin and maturation process of B-cell precursors.

    <p>B-cell precursors originate from stem cells in the fetal liver and migrate to the bone marrow, where they undergo maturation, which does not require the thymus.</p> Signup and view all the answers

    What is the significance of TCR-peptide-MHC recognition in T cell activation?

    <p>TCR-peptide-MHC recognition is essential for T cell activation, enabling the T cell to recognize specific antigens and initiate immune response mechanisms.</p> Signup and view all the answers

    Explain the role of memory T and B cells after an infection is cleared.

    <p>Memory T and B cells, derived from their activated counterparts, persist in the body to ensure a rapid and robust immune response upon reinfection with the same pathogen.</p> Signup and view all the answers

    What distinguishes the function of effector/helper T cells from regulatory T cells?

    <p>Effector/helper T cells (Teff) coordinate immune responses, while regulatory T cells (Treg) are primarily involved in suppressing excessive inflammation and preventing autoimmune reactions.</p> Signup and view all the answers

    How does the differentiation of B cells into plasma cells contribute to the immune response?

    <p>B cells differentiate into plasma cells that produce antibodies, which help neutralize pathogens and facilitate their elimination from the body.</p> Signup and view all the answers

    What initiates the clonal selection process in T and B cells during their maturation?

    <p>Clonal selection is initiated when immature T and B cells successfully bind to specific antigens through their receptors during their maturation process.</p> Signup and view all the answers

    What is the function of the Fc fragment in an antibody?

    <p>The Fc fragment facilitates the attachment of antibodies to host cells via Fc receptors and is essential for complement activation.</p> Signup and view all the answers

    How do plasma cells differ from memory cells in terms of lifespan and function?

    <p>Plasma cells have a shorter lifespan and actively produce antibodies, while memory cells survive longer in the absence of antigen and remain functionally inactive until re-exposed to the same antigen.</p> Signup and view all the answers

    Describe the significance of positive selection during B-cell maturation.

    <p>Positive selection ensures that only B cell precursors capable of generating a functional BCR survive, allowing them to mature into B cells.</p> Signup and view all the answers

    What triggers negative selection of B cells, and why is it important?

    <p>Negative selection is triggered when a BCR strongly binds to self-antigens, leading to apoptosis of those B cells.</p> Signup and view all the answers

    What role does the IgM BCR play in the positive selection of pre-B cells?

    <p>The IgM BCR allows pre-B cells to pass through positive selection, indicating their readiness to mature into functional B cells.</p> Signup and view all the answers

    In what ways do memory cells contribute to secondary immune responses?

    <p>Memory cells respond rapidly upon re-encountering their specific antigen, initiating a quicker and stronger immune response than the primary response.</p> Signup and view all the answers

    Explain how the Fc region of antibodies is involved in forming multimers of IgM and IgA.

    <p>The Fc region is crucial for linking multiple antibody units together, allowing IgM and IgA to form larger multimeric structures.</p> Signup and view all the answers

    What is the significance of the BCR's ability to bind a variety of antigens during B cell maturation?

    <p>The BCR's versatility enables B cells to interact with a wide range of antigens, which is essential for the adaptive immune system's ability to recognize diverse pathogens.</p> Signup and view all the answers

    The first phase of B cell maturation is called the antigen-______ phase.

    <p>independent</p> Signup and view all the answers

    Bruton’s tyrosine kinase (BTK) is essential for the differentiation of pre-B cells into ______.

    <p>B cells</p> Signup and view all the answers

    Mature B cells possess functional ______ receptors that interact with antigens.

    <p>antigen</p> Signup and view all the answers

    Each antibody molecule has a characteristic ______ shape.

    <p>Y</p> Signup and view all the answers

    The antigen binding fragment of an antibody is also known as ______.

    <p>Fab</p> Signup and view all the answers

    Antibodies consist of light (L) and heavy (H) polypeptide ______.

    <p>chains</p> Signup and view all the answers

    The regions of the polypeptide chains that bind the antigen are composed of hypervariable ______.

    <p>amino acids</p> Signup and view all the answers

    During the second phase of B cell maturation, mature B cells are activated upon exposure to ______.

    <p>antigens</p> Signup and view all the answers

    Only B cells that do not strongly bind self-antigens are allowed to leave the ______ and will be self-tolerant.

    <p>bone marrow</p> Signup and view all the answers

    The effector cells in the B lymphocyte are antibody-secreting cells, called ______ cells.

    <p>plasma</p> Signup and view all the answers

    Natural killer (NK) cells are known to kill virus-infected cells using perforin and ______.

    <p>granzyme</p> Signup and view all the answers

    The Fc fragment of an antibody is necessary for transport of IgA across ______ barriers.

    <p>epithelial</p> Signup and view all the answers

    Plasma cells release antibodies in response to ______.

    <p>antigens</p> Signup and view all the answers

    Mature naïve B cells express BCR IgM and IgD immunoglobulin on their ______.

    <p>surface</p> Signup and view all the answers

    The process of clonal selection begins with ______ selection for B-cell precursors.

    <p>positive</p> Signup and view all the answers

    The primary role of helper T cells is in the coordination and regulation of the adaptive ______ response.

    <p>immune</p> Signup and view all the answers

    When a BCR strongly binds a self-antigen during negative selection, the cell undergoes ______.

    <p>apoptosis</p> Signup and view all the answers

    Memory cells survive for long periods in the absence of ______.

    <p>antigen</p> Signup and view all the answers

    IgM and IgA antibodies are fused together into larger ______ using the Fc region.

    <p>multimers</p> Signup and view all the answers

    The main function of cytotoxic T cells is to recognize and kill any cell that has foreign ______ on its surface.

    <p>proteins</p> Signup and view all the answers

    The BCR can bind to various circulating proteins, lipids, carbohydrates, or ______.

    <p>nucleic acids</p> Signup and view all the answers

    CD4 positive T cells can be divided into three main categories: effector/helper cells, follicular helper cells, and ______ T cells.

    <p>regulatory</p> Signup and view all the answers

    B cells can differentiate into plasma cells that produce ______.

    <p>antibodies</p> Signup and view all the answers

    Once a B cell becomes a mature plasma cell, it can release up to ______ antibodies per second.

    <p>2,000</p> Signup and view all the answers

    T-cell precursors arise from common lymphoid ______.

    <p>progenitors</p> Signup and view all the answers

    B-cell precursors migrate to the bone marrow, which is their main location during ______ and adult life.

    <p>childhood</p> Signup and view all the answers

    All T cells require ______ interactions and TCR-peptide-MHC recognition for their functions.

    <p>cell-cell</p> Signup and view all the answers

    CD8 positive T cells are specifically referred to as ______ T lymphocytes.

    <p>cytotoxic</p> Signup and view all the answers

    The ability of a B-cell precursor to make its antigen receptor determines whether it is allowed to develop into a mature ______.

    <p>B cell</p> Signup and view all the answers

    T lymphocytes mature in the ______.

    <p>thymus</p> Signup and view all the answers

    B lymphocytes are responsible for producing ______.

    <p>antibodies</p> Signup and view all the answers

    Naïve lymphocytes are those that have not yet encountered a specific ______.

    <p>antigen</p> Signup and view all the answers

    The development of B cells occurs mainly in the ______.

    <p>bone marrow</p> Signup and view all the answers

    T cell precursors are derived from common lymphoid ______.

    <p>progenitors</p> Signup and view all the answers

    Adaptive immunity is mediated by T and B ______.

    <p>lymphocytes</p> Signup and view all the answers

    Memory cells arise from activated T and B ______.

    <p>lymphocytes</p> Signup and view all the answers

    B lymphocytes are primarily responsible for ______ immunity.

    <p>humoral</p> Signup and view all the answers

    Match the phases of B cell maturation to their descriptions:

    <p>Antigen-independent phase = Involves stem cells, pre-B cells, and B cells Antigen-dependent phase = Mature B cells interact with antigens Pre-B cells differentiation = Requires a functional immunoglobulin on the cell surface X-linked agammaglobulinemia = Condition due to mutation in Bruton’s tyrosine kinase</p> Signup and view all the answers

    Match the components of an antibody to their characteristics:

    <p>Heavy chains = Molecular weight of 50,000 to 70,000 Light chains = Molecular weight of about 25,000 Fab region = Antigen binding fragment consisting of variable and constant regions Stalk region = Composed of constant regions of the heavy chains</p> Signup and view all the answers

    Match the antibody parts to their specific functions:

    <p>Variable region (VL) = Binds to the antigen with high specificity Constant region (CL) = Maintains the structure of the light chain First constant region of heavy chain (CH1) = Part of the Fab region Constant regions (CH2, etc.) = Form the stalk of the antibody molecule</p> Signup and view all the answers

    Match the following T cell structures with their primary characteristics:

    <p>TCR α chain = Single chain that includes a variable region and constant region TCR β chain = Single chain that binds to the peptide–MHC complex CD3ζ = Transmits TCR recognition signal inside the cell CD4 = Coreceptor that binds to MHC class II</p> Signup and view all the answers

    Match the characteristics of the B cell receptor (BCR) with their associated descriptions:

    <p>IgM molecule = Primary immunoglobulin on mature B cells BCR tethering = Done by an additional region at the end of the heavy chain Maturation = Involves interactions with antigens for activation Circulating lifespan = Short unless activated through antigen receptors</p> Signup and view all the answers

    Match the selection processes of T-cell precursors with their outcomes:

    <p>Positive selection = Survival of T-cell precursors that bind MHC Negative selection = Apoptosis of T-cells with strong self-antigen contact Double-negative T cells = Initial state lacking CD4, CD8, and TCR Mature naïve T cells = Exit thymus after successful selection</p> Signup and view all the answers

    Match the components of B cell development to their respective roles:

    <p>Bruton’s tyrosine kinase (BTK) = Signals for pre-B cell differentiation Immunoglobulin gene recombination = Creates unique antigen receptors Stem cells = Initial precursor cells for B cell development Mature B cells = Require antigen interaction for sustained lifespans</p> Signup and view all the answers

    Match the T cell characteristics with their definitions:

    <p>Naïve T cells = T cells that have not encountered antigens Effector T cells = Activated T cells that perform immune functions Memory T cells = Long-lived T cells that respond to previously encountered antigens Cytotoxic T lymphocytes = T cells that kill infected or cancerous cells</p> Signup and view all the answers

    Match the immunoglobulin structural elements to their functions:

    <p>Y-shaped structure = General form of antibody molecules Antigen-binding sites = Located at the ends of the Fab regions Hypervariable amino acid sequences = Create variability for antigen specificity Stalk composed of constant regions = Determines effector functions of the antibody</p> Signup and view all the answers

    Match the components involved in T cell activation with their roles:

    <p>MHC = Presents peptides to TCR CD8 = Coreceptor for cytotoxic T cells Tyrosine kinases = Phosphorylate downstream messengers Transmembrane domain = Binds the TCR chains to the cell membrane</p> Signup and view all the answers

    Match the terms related to antibody structure with their definitions:

    <p>Fab = Fragment that binds the antigen in the antibody Fc = Fragment that mediates effector functions H chains = Heavy polypeptide chains in the antibody L chains = Light polypeptide chains that accompany heavy chains</p> Signup and view all the answers

    Match the types of B cells with their respective characteristics:

    <p>Plasma cells = B cells that produce antibodies Mature B cells = In circulation and require antigen activation Memory B cells = Long-lived and respond to re-exposure of antigens Pre-B cells = Stage before the differentiation into mature B cells</p> Signup and view all the answers

    Match the components of T cell receptor (TCR) structure with their attributes:

    <p>Variable region = Part of TCR that binds to peptides Constant region = Region that provides structural stability Hypervariable region = Contains sequences critical for antigen recognition Cytoplasmic tail = Binds to CD3 for signal transmission</p> Signup and view all the answers

    Match the following T cell types with their specific functions:

    <p>CD4+ T cells = Assist in orchestrating immune responses CD8+ T cells = Directly kill infected or cancerous cells Regulatory T cells = Suppress immune responses and maintain tolerance Memory T cells = Rapidly respond upon re-encountering antigens</p> Signup and view all the answers

    Match the following terms related to T cell maturation with their descriptions:

    <p>Thymus cortex = Site of positive selection in T cell development Thymus medulla = Site where negative selection occurs Double-positive T cells = Cells expressing both CD4 and CD8 Common lymphoid progenitors = Cells that give rise to T-cell precursors</p> Signup and view all the answers

    Match the outcomes of T cell selection with the corresponding processes:

    <p>Strong binding to self-antigens = Leads to apoptosis in negative selection Binding to MHC with moderate affinity = Leads to survival in positive selection Failure to bind MHC = Results in cell death during positive selection Weaker self-antigen binding = Allows exit as mature naïve T cells</p> Signup and view all the answers

    Match the following terms with their definitions:

    <p>Naïve lymphocyte = Lymphocyte that has not encountered a specific antigen Plasma cell = Differentiated B cell that produces antibodies Memory cell = Long-lived lymphocyte that remembers past infections TCR = Receptor on T cells that recognizes specific antigens</p> Signup and view all the answers

    Match the following lymphocyte types with their primary roles:

    <p>T lymphocyte = Directly attack infected or tumor cells B lymphocyte = Produce antibodies to target pathogens NK cell = Kill infected or cancerous cells without prior sensitization Regulatory T cell = Help modulate the immune response</p> Signup and view all the answers

    Match the following locations with the corresponding lymphocyte development:

    <p>Thymus = Site for T cell maturation Bone marrow = Primary site for B cell development Peripheral lymphoid organs = Location for lymphocyte activation Fetal liver = Initial site of lymphocyte production</p> Signup and view all the answers

    Match the following processes with their outcomes:

    <p>Clonal selection = Activation and proliferation of specific lymphocytes Thymic selection = Elimination of self-reactive T cells Antigen presentation = Display of antigens to T cells by APCs Isotype switching = Change in antibody class produced by B cells</p> Signup and view all the answers

    Match the following proteins with their associated lymphocyte:

    <p>CD4 = Marker for helper T cells CD8 = Marker for cytotoxic T cells BCR = Receptor found on B lymphocytes TCR = Receptor used by T lymphocytes for antigen recognition</p> Signup and view all the answers

    Match the following antibody characteristics with their descriptions:

    <p>Fab region = Part of the antibody that binds to the antigen Fc region = Part of the antibody that interacts with immune cells Heavy chain = Polypeptide chain that contributes to the antibody's structure Light chain = Smaller polypeptide chain that pairs with the heavy chain</p> Signup and view all the answers

    Match the following immune functions with their respective lymphocytes:

    <p>B lymphocytes = Humoral immunity through antibody production Cytotoxic T cells = Kill infected and cancerous cells directly Helper T cells = Coordinate immune responses Memory B cells = Provide long-term immunity by remembering past infections</p> Signup and view all the answers

    Match the following types of cells with their primary mechanism:

    <p>Plasma cells = Secrete antibodies into the bloodstream Regulatory T cells = Suppress overactive immune responses Memory T cells = Respond rapidly upon re-exposure to antigens Natural Killer (NK) cells = Induce apoptosis in infected host cells</p> Signup and view all the answers

    Match the following antibody components with their primary functions:

    <p>Fc = Attaches antibody to host cells Fab = Binds the antigen IgA = Transport across epithelial barriers IgG = Transport from mother to fetus</p> Signup and view all the answers

    Match the following B cell types with their characteristics:

    <p>Plasma cells = Release up to 2,000 antibodies per second Memory cells = Survive long periods in the absence of antigens Pre-B cells = Lack surface BCR Mature B cells = Have surface IgM after positive selection</p> Signup and view all the answers

    Match the phases of B cell development with their corresponding processes:

    <p>Positive selection = Survival of cells generating functional BCR Negative selection = Removal of self-reactive B cells via apoptosis Clonal selection = Process involving antigen-stimulated B cells BCR rearrangement = Generation of functional immunoglobulin gene segments</p> Signup and view all the answers

    Match the following terms related to B cell receptors to their meanings:

    <p>BCR = B cell receptor capable of binding circulating antigens Self-antigen = Body's own proteins that BCR may encounter Autoreactivity = Potential reaction of B cells to self-antigens Immunoglobulin gene segments = DNA sequences rearranged to form BCR</p> Signup and view all the answers

    Match the following immunological concepts with their descriptions:

    <p>Clonal selection = Survival and activation of specific B cell clones Plasmacytes = Another name for plasma cells Effector functions = Actions performed by plasma cells upon antigen encounter Secondary immune response = Rapid response by memory cells to previously encountered antigens</p> Signup and view all the answers

    Match the following antibody types with their associated characteristics:

    <p>IgM = Forms complexes during immune responses IgA = Main antibody in mucosal immunity IgG = Most abundant antibody in circulation IgE = Involved in allergic reactions and responses to parasites</p> Signup and view all the answers

    Match the following terms related to B cell maturation with their characteristics:

    <p>Surface IgM = Present after successful positive selection Self-antigens = Targets encountered during immature B cell stage Apoptosis = Process eliminating self-reactive B cells Functional BCR = Hindered by failed gene rearrangement</p> Signup and view all the answers

    Match the following immune cell types with their respective functions:

    <p>Plasma cells = Secrete antibodies to neutralize pathogens Memory B cells = Facilitate rapid response upon re-exposure to antigens B cell precursors = Undergo maturation to develop BCR Naive B cells = Have yet to encounter their specific antigen</p> Signup and view all the answers

    Match the following T cell types with their primary functions:

    <p>Th-1 cells = Coordinate immune response against intracellular pathogens Th-2 cells = Assist in antibody production and responses to extracellular allergens Th-17 cells = Promote responses against extracellular bacteria and fungi Regulatory T (Treg) cells = Suppress inflammation and maintain tolerance</p> Signup and view all the answers

    Match the following B cell functions with their descriptions:

    <p>Plasma cells = Produce antibodies Memory B cells = Respond quickly to reinfection B-cell receptor (BCR) = Antigen receptor on B cells Differentiation = Process of becoming functionally specialized cells</p> Signup and view all the answers

    Match the following T cell surface markers with their corresponding T cell types:

    <p>CD4 = Helper T cells CD8 = Cytotoxic T cells CD25 = Regulatory T cells CD279 = Inhibitory marker on T cells</p> Signup and view all the answers

    Match the following categories of CD4 positive T cells with their primary roles:

    <p>Effector/helper T (Teff) cells = Mediate adaptive immune responses Follicular helper T (Tfh) cells = Assist B cells in antibody development Regulatory T (Treg) cells = Control autoimmune responses Naïve T cells = Await activation</p> Signup and view all the answers

    Match the following parts of the immune response with their related cell types:

    <p>Antibody production = B cells Cytotoxic activity = CD8 positive T cells T helper activation = CD4 positive T cells Memory response = Memory B and T cells</p> Signup and view all the answers

    Match the following phases of T cell development with their descriptions:

    <p>Thymic selection = Process of generating mature naïve T cells Positive selection = Selection for T cells with moderate affinity for self-MHC Negative selection = Elimination of self-reactive T cells Activation = T cell response initiation upon antigen encounter</p> Signup and view all the answers

    Match the following concepts related to T and B lymphocytes:

    <p>Clonal selection = Mechanism for expanding specific lymphocyte clones Self-tolerance = Prevention of autoimmunity Antigen specificity = Unique recognition by TCR or BCR Memory formation = Long-term protection against reinfection</p> Signup and view all the answers

    Match the following terms with their definitions:

    <p>Cytotoxic T lymphocytes (CTLs) = Recognize and kill infected cells Antigen receptor = Molecule that binds specific antigens Immunoglobulin = Antibody protein produced by plasma cells Cluster of differentiation (CD) = Surface proteins used to identify cell types</p> Signup and view all the answers

    Study Notes

    Lymphocyte Differentiation

    • Naïve lymphocytes differentiate into either effector cells or memory cells.
    • This differentiation is initiated by antigen recognition.
    • The immune response that develops is specific to the antigen.

    T-cell Receptor Structure

    • TCRs have two chains instead of four, an alpha (α) chain and a beta (β) chain.
    • Each chain has a variable region that binds to the peptide-MHC complex and a constant region.
    • The variable region includes the hypervariable region.
    • The α and β chains are located outside the cell and attached to the cell membrane.
    • The cytoplasmic tail of the TCR binds to the CD3ζ protein.
    • CD3 proteins are essential for signal transduction from the TCR to the cell's interior.
    • The CD4 and CD8 proteins are coreceptors for the TCR, binding to MHC class II and class I, respectively.
    • The cytoplasmic domains of CD4 and CD8 amplify the TCR signal.

    Thymic Selection

    • Positive selection: T-cell precursors in the thymus cortex lack CD4 and CD8 and a TCR.
      • They become CD4+CD8+ (double-positive) while rearranging their TCR genes to generate unique TCRs.
      • Double-positive cells encounter specialized thymus cells expressing self-peptides bound to MHC.
      • Only T-cell precursors that bind to MHC survive.
    • Negative selection: Surviving cells move to the thymus medulla.
      • Cells that bind strongly to self-antigens are killed by apoptosis.
      • Cells that don't bind strongly to self-peptides survive.
    • Mature naïve T cells exit the thymus and circulate through secondary lymphoid tissues searching for antigens.

    T Lymphocyte Subtypes

    • T lymphocytes can be divided based on cell surface markers called "cluster of differentiation" (CD).
    • CD4+ T cells (Helper T cells) can be divided into:
      • Effector/helper (Teff or Th) cells leave the lymphoid organ to coordinate immune responses in inflamed tissue.
      • Follicular helper (Tfh) cells move into B-cell follicles of lymphoid organs to help B cells.
      • Regulatory T (Treg) cells suppress inflammation.
    • Effector/helper (Teff or Th) cells are further divided into:
      • Th1 cells
      • Th2 cells
      • Th17 cells
    • CD8+ T cells (Cytotoxic T lymphocytes, CTLs) recognize and kill cells displaying foreign proteins on their surface.
      • This occurs when cells are infected by viruses, bacteria, or fungi.
    • T cells require cell-cell interactions and TCR-peptide-MHC recognition for both initial priming and effector functions.
    • After infection clearance, all T-cell types contribute clones to the memory T-cell pool, providing rapid response to reinfection.

    B Lymphocytes

    • B cells have two key functions:
      • Differentiation into plasma cells that produce antibodies (immunoglobulins).
      • Development into long-lived memory B cells that rapidly respond to reinfection.
    • The immunoglobulin on the B-cell surface is its antigen receptor (B-cell receptor or BCR).
    • The ability of a B-cell precursor to produce a functional BCR determines its maturation.

    Origin and Maturation of B Cells

    • B-cell precursors arise from stem cells in the fetal liver.
    • By birth, these stem cells migrate to the bone marrow, their primary location during childhood and adulthood.
    • Unlike T cells, B cells mature without requiring the thymus.
    • B-cell maturation has two phases:
      • Antigen-independent phase: Stem cells, pre-B cells, and B cells recombine their immunoglobulin genes to make a unique antigen receptor.
        • A functional immunoglobulin must be on the cell surface for a pre-B cell to differentiate into a B cell.
        • Bruton's tyrosine kinase (BTK) detects this immunoglobulin and signals for continued cell division and differentiation.
        • Mutations in BTK cause X-linked agammaglobulinemia, preventing cells from progressing to the pre-B stage and antibody production.
      • Antigen-dependent phase: Mature B cells with functional antigen receptors interact with antigens.

    Mature B Cells with BCR

    • The immunoglobulin (Ig) or BCR of a mature B cell is an IgM molecule tethered to the B-cell surface.
    • Many B cells are produced daily, but only a small fraction make it from the bone marrow to circulation.
    • Without antigen receptor activation, circulating B cells have short lifespans (days or weeks).

    Antibody (Immunoglobulin) Structure

    • Antibodies are glycoproteins composed of light (L) and heavy (H) polypeptide chains.
    • The simplest antibody molecule has a Y shape with two identical H chains and two identical L chains.
    • The Y's end is composed of two identical antigen-binding fragments (Fabs).
    • The Fab includes the variable regions of the L chain (VL) and H chain (VH), and the constant regions of the L chain (CL) and the first constant region of the H chain (CH1).
    • The antigen-binding portions of the L and H chains are 5-10 amino acids long, each composed of three hypervariable amino acid sequences.
    • The Y's other end is a single stalk, called the constant or "crystallizable" fragment (Fc), made of the remaining constant regions of the H chains (CH2, etc.).
    • The Fc attaches the antibody to host cells, complement, and helps form multimers of IgM and IgA, as well as transport of IgA across epithelial barriers and IgG across the placenta.

     ### Plasma and Memory Cells

    • Plasma cells release antibodies in response to antigens, up to 2,000 antibodies per second.
    • Plasma cells are also called plasmacytes or effector cells and have shorter lifespans than memory cells.
    • Memory cells, generated from antigen-stimulated lymphocytes, survive for long periods without antigen.
    • Memory cells are functionally inactive unless stimulated by antigen.
    • When encountering the same antigen, memory cells rapidly respond to initiate secondary immune responses.

    Clonal Selection

    • The first step in B-cell clonal selection is called positive selection.
      • Pre-B cells lack surface BCR.
      • If a B-cell precursor fails to rearrange its immunoglobulin gene segments and produce a functional BCR, it dies.
      • This is positive selection because only cells that generate a BCR survive and mature.
    • Pre-B cells that successfully generate surface IgM pass through positive selection and become B cells.
    • At this stage, their IgM BCRs encounter self-antigens.
      • BCR can bind to any protein, lipid, carbohydrate, or nucleic acid, unlike T-cell receptors which bind only to peptides complexed with MHC proteins.
      • This phase occurs in the bone marrow, ensuring that B cells encounter mostly self-antigens.
    • This phase, called negative selection, removes B cells with BCRs that strongly bind self-antigens, preventing autoimmunity.
      • Only B cells that do not strongly bind self-antigens leave the bone marrow and are considered self-tolerant.

    Comparison of T Cells and B Cells

    Feature T Cells B Cells
    Antigen Recognition Peptide-MHC complexes Diverse antigens, including proteins, lipids, carbohydrates, and nucleic acids
    Receptor TCR BCR (immunoglobulin)
    Function Helper T cells: coordinate immune responses; Cytotoxic T cells: kill infected or cancerous cells Antibody production; Memory B cell formation
    Maturation Thymus Bone marrow

    Functions of T Cells

    • Host defense against infection (especially Mycobacterium tuberculosis, fungi, and virus-infected cells)
    • Tumor rejection
    • Coordination and regulation of adaptive immune response (helper T cells)
    • Allergy/hypersensitivity (e.g., poison oak)
    • Autoimmunity
    • Transplant graft rejection

    Functions of B Cells

    • Host defense against infection (opsonize bacteria, neutralize toxins and viruses)
    • Allergy/hypersensitivity (e.g., hay fever, anaphylactic shock)
    • Autoimmunity

    Natural Killer (NK) Cells

    • Large granular lymphocytes (Non-B, non-T lymphocytes).
    • Lack T-cell receptor, CD3 proteins, and surface IgM and IgD.
    • Do not require the thymus for development.
    • Normal numbers in severe combined immunodeficiency disease (SCID) patients.
    • Activity not enhanced by prior exposure.
    • Have no memory.

    NK Cell Function

    • Recognize virus-infected cells by detecting the absence of class I MHC proteins on their surface.
    • Kill virus-infected and cancer cells using perforin and granzyme.
    • Killing is nonspecific and independent of foreign antigen presentation by class I or II MHC proteins.
    • Produce gamma interferon that activates macrophages to kill ingested bacteria.

    Summary

    • T and B lymphocytes are crucial components of adaptive immunity.
    • Mature naïve T cells have unique TCRs.
    • Mature naïve B cells have BCR IgM and IgD immunoglobulins.
    • Naïve lymphocytes have never encountered a specific antigen.
    • The effector cells in B lymphocytes are antibody-secreting cells called plasma cells.
    • The effector cells in T lymphocytes are helper and cytotoxic T lymphocytes.

    Lymphocyte Origin and Maturation

    • T lymphocytes (T cells): Develop from common lymphoid progenitors and mature in the thymus
      • Thymus: specialized organ where T cells mature and express unique T cell receptors (TCRs)
      • Naïve T cells: Mature T cells that have not encountered foreign antigens
      • TCR: composed of an alpha (α) and beta (β) chain
      • CD3: protein associated with TCR that transmits signals inside the cell
      • CD4 and CD8: Co-receptors that bind to MHC class II and I respectively, amplifying TCR signal transmission
    • B lymphocytes (B cells): Develop from stem cells in the fetal liver and later in the bone marrow
      • Bone marrow: Primary location for B cell maturation in adults
      • Antigen-Independent Phase: Development of stem cells to pre-B cells and then B cells
        • Rearrangement of immunoglobulin genes to make unique antigen receptors
        • Bruton's tyrosine kinase (BTK): Protein that detects functional immunoglobulin and signals for cell division and differentiation
        • Mutation in BTK gene leads to X-linked agammaglobulinemia, where B cell development is arrested, leading to a lack of antibodies
      • Antigen-Dependent Phase: Mature B cells with functional antigen receptors interact with antigens
      • B cell Receptor (BCR): Surface immunoglobulin (Ig) on mature B cells, mainly an IgM molecule
        • Tethered to the cell surface via a region at the end of its heavy chain

    T Cell Receptor Structure

    • Composed of a single α and a single β chain
    • Contains both variable and constant regions
      • Variable region: Includes the hypervariable region that binds to the peptide-MHC complex
      • Constant region: Connects α and β chains and anchors the receptor to the cell membrane
    • Transmembrane domain: Anchors the α and β chains to the cell membrane
    • Cytoplasmic tail: Binds to the CD3ζ molecule
    • CD3 proteins: Essential for signal transmission from the TCR to the inside of the cell

    T Cell Selection

    • Positive selection: Within the thymus cortex
      • Double-negative T cell precursors: Lack CD4 and CD8, and TCR
      • Double-positive T cells: Express CD4 and CD8 and undergo TCR gene rearrangement
      • Interaction with MHC: T cell precursors that bind to MHC survive, others die by apoptosis
    • Negative selection: Within the thymus medulla
      • Strong binding to self-antigens: T cells are eliminated by apoptosis
      • Weak binding to self-antigens: T cells survive and exit the thymus as mature naïve T cells

    T Cell Types

    • CD4 positive T cells (Helper T cells):
      • Effector/Helper (Teff or Th cells): Coordinate immune responses in inflamed tissues
      • Follicular helper (Tfh cells): Help B cells in lymphoid organs
      • Regulatory T (Treg cells): Suppress inflammation
      • Th1 cells: Secrete IFN-γ, IL-2, and TNF-α
      • Th2 cells: Secrete IL-4, IL-5, and IL-13
      • Th17 cells: Secrete IL-17, IL-21, and IL-22
    • CD8 positive T cells (Cytotoxic T lymphocytes, CTLs): Recognize and kill cells displaying foreign proteins on their surface
      • Target cells: Infected cells, tumor cells

    B Cell Functions

    • Plasma cell differentiation: Release antibodies, also known as immunoglobulins
    • Memory cell formation: Long lived cells that rapidly respond to reinfection

    Antibody (Immunoglobulin) Structure

    • Glycoproteins composed of light (L) and heavy (H) chains
    • Basic Y-shaped structure with two identical light and two identical heavy chains
    • Antigen-binding Fragment (Fab): Composed of variable and constant regions of both light and heavy chains, responsible for antigen binding
      • Variable region: Contains hypervariable regions that directly bind to the antigen
      • Constant region: Provides structure and stability to the Fab
    • Constant Fragment (Fc): Stalk region composed of heavy chain constant regions
      • Provides functions like attachment to host cells, complement activation, and multimer formation
      • Fc receptors: Found on specific cell types, allowing for interaction with antibodies

    Plasma and Memory Cells

    • Plasma cells: Secrete large amounts of antibodies in response to antigens
      • Short lifespan compared to memory cells
    • Memory cells: Long-lived cells generated from antigen-stimulated lymphocytes
      • Remain inactive unless stimulated by antigen
      • Rapidly respond to reinfection, initiating secondary immune responses

    Clonal Selection

    • Positive selection: Early stage where pre-B cells lack surface BCR
      • Pre-B cells that fail to generate a functional BCR die
      • Only cells with a functional BCR survive and mature further
    • Negative selection: Mature B cells with surface IgM encounter self-antigens
      • B cells that strongly bind self-antigens are removed by apoptosis
      • This ensures that B cells do not attack self-antigens, preventing autoimmune disorders

    Lymphocytes

    • Lymphocytes are a type of white blood cell.
    • Lymphocytes play an important role in the immune system, which helps the body fight disease and infection.
    • There are two main types of lymphocytes: T lymphocytes (T cells) and B lymphocytes (B cells).
    • T cells control the body's immune system response and directly attack and kill infected cells and tumor cells.
    • B cells make antibodies, which are proteins that target viruses, bacteria and other foreign invaders.

    T Lymphocytes

    • T lymphocyte precursors (immature T cells) derive from common lymphoid progenitors.
    • T cell development includes a step in which the precursors migrate through the thymus.
    • It is during passage through the thymus that a T-cell precursor begins to express a unique TCR.
    • Mature naïve T cells are T cells that have never encountered a foreign antigen.

    Naïve Lymphocytes

    • Naïve lymphocytes are lymphocytes that have not encountered a specific antigen.
    • All lymphocytes that leave the central lymphoid organs are naïve.
    • Naïve lymphocytes differentiate into either effector cells or memory cells, which is initiated by antigen recognition.

    T Cell Receptor Structure

    • TCRs have analogous chains but only have two chains instead of four.
    • The TCR is composed of a single α (alpha) chain and a single β (beta) chain.
    • Each chain includes a variable region, which includes the hypervariable region that binds to the peptide–MHC complex, and a constant region, which attaches the α chain and β chain to each other.
    • The α chain and β chain are mostly located outside the cell, and they are fixed to the cell membrane by a transmembrane domain and a short cytoplasmic tail.
    • The tail binds to a molecule called CD3ζ (CD3-zeta).
    • All T cells have CD3 proteins in association with TCR.
    • The purpose of CD3 is to transmit the TCR peptide recognition signal from the surface to the inside of the cell.
    • CD4 and CD8 proteins are coreceptors for the TCR.
    • They sit in the T-cell membrane and bind to nonpolymorphic regions on MHC (class II and class I, respectively).
    • The cytoplasmic domains of CD4 and CD8 amplify the TCR signal transmission.

    Thymic Selection

    • Positive selection:
      • T-cell precursors arrive in the thymus cortex lacking CD4 and CD8 (double-negative) and lacking a T-cell receptor (TCR).
      • They then become CD4+CD8+ (double-positive) while rearranging their TCR genes to generate unique TCRs.
      • Next, double-positive cells meet specialized thymus cells expressing a wide range of self-peptides complexed with class I and class II major histocompatibility complex (MHC).
      • Only the T cell precursors that do bind to MHC are chosen to survive.
    • Negative selection:
      • The surviving cells move into the thymus medulla, where those that make strong contacts with self-antigens are killed by apoptosis.
      • The T cells that don’t strongly bind to self peptides are allowed to survive.
      • These cells exit the thymus as mature naïve T cells and migrate throughout the circulation and secondary lymphoid tissues surveying for antigen.

    T Lymphocyte Classification

    • T lymphocyte cells can be further divided based on their function and based on molecules on the cell surface called “cluster of differentiation” (or CD).
    • CD4 positive T-cell can be divided into three main categories: (1) effector/helper (Teff or Th) cells, which leave the lymphoid organ and coordinate immune responses in inflamed tissue; (2) follicular helper (Tfh) cells, which move into the B-cell follicle of the lymphoid organ and help the B cells; and (3) regulatory T (Treg) cells, which suppress inflammation.
    • CD8 positive marks the cells that are called cytotoxic T lymphocytes (CTLs).
    • The main function of cytotoxic (CD8-positive) T cells is to recognize and kill any cell that has foreign (non-self) proteins on its surface.
    • Cells might contain foreign proteins because the cells have been infected by viruses, bacteria, or fungi.
    • After an infection is cleared, each of these T-cell types can contribute clones to the pool of memory T cells that patrol the body and respond rapidly to reinfection.

    B Lymphocytes

    • B cells perform two important functions:
      • They differentiate into plasma cells that produce antibodies (also called immunoglobulins).
      • They can become long-lived memory B cells that can rapidly respond to a reinfection.
    • The immunoglobulin on the B-cell surface is its antigen receptor (B-cell receptor or BCR).
    • The ability of a B-cell precursor to make this antigen receptor determines whether it is allowed to develop into a mature B cell.

    Origin of B Cells

    • B-cell precursors first arise from stem cells in the fetal liver, but by the time of birth, these stem cells migrate to the bone marrow, which is their main location during childhood and adult life.
    • Unlike T cells, B cells do not require the thymus for maturation.
    • The maturation of B cells has two phases:
      • The first is the antigen-independent phase, which consists of stem cells, pre-B cells, and B cells, and it is during this phase that the B cell recombines its immunoglobulin genes to make a unique antigen receptor.
      • During the second phase, which is the antigen-dependent phase, mature B cells with functional antigen receptors interact with antigens.

    Antibody Structure

    • Antibodies are glycoproteins made up of light (L) and heavy (H) polypeptide chains.
    • The terms light and heavy refer to molecular weight; light chains have a molecular weight of about 25,000, whereas heavy chains have a molecular weight of 50,000 to 70,000.
    • The simplest antibody molecule has a Y shape and consists of four polypeptide chains: two identical H chains and two identical L chains.
    • One end of the Y is composed of two identical pieces that bind the antigen, and therefore, this is called the antigen binding fragment (or Fab).
    • The Fab includes the variable region of the L chain (VL) and the variable region of the H chain (VH), as well as the constant region of the L chain (CL) and the first constant region of the H chains (CH1).
    • The portions of the L and H chains that actually bind the antigen are only 5 to 10 amino acids long, each composed of three extremely variable (hypervariable) amino acid sequences.
    • The other end of the Y is a single stalk, where the H chains come together, and it is made of the remaining three or four constant regions of each of the H chains (CH2, etc.).
    • This is called the constant or “crystallizable” fragment (or Fc).
    • The Fc is needed to attach the antibody to host cells (e.g., via Fc receptors) or to complement (at the CH2 domain).
    • The Fc is also the region that is used to fuse IgM and IgA together into larger “multimers.”
    • It is also necessary for transport of IgA across epithelial barriers and transport of IgG from mother to fetus through the placenta.

    Plasma and Memory Cells

    • Plasma cells release antibodies in response to antigens.
    • Once a B cell becomes a mature plasma cell, it can release up to 2,000 antibodies per second.
    • Plasma cells are also called plasmacytes or effector cells.
    • They have a shorter lifespan than memory cells.
    • Memory cells, also generated from the progeny of antigen-stimulated lymphocytes, do survive for long periods in the absence of antigen.
    • Memory cells are functionally inactive; they do not perform effector functions unless stimulated by antigen.
    • When memory cells encounter the same antigen that induced their development, the cells rapidly respond to initiate secondary immune responses.

    Clonal Selection

    • The first step of B-cell clonal selection is called positive selection.
    • Pre-B cells lack surface BCR.
    • If a B-cell precursor fails to rearrange its immunoglobulin gene segments and generate a functional BCR, it dies before it reaches the mature B-cell stage.
    • This is called positive selection because only those cells that do generate a BCR are allowed to survive and mature.
    • Pre-B cells that do successfully generate surface IgM pass through positive selection and progress to become B cells.
    • At this stage, their IgM BCRs immediately encounter self-antigens.
    • During this phase, called negative selection, if the BCR strongly binds a self-antigen, this indicates high potential for autoreactivity.
    • This cell will be removed from the pool of mature B –cell clone by apoptosis.

    Lymphocyte Origin & Maturation

    • T-cell precursors develop from common lymphoid progenitors, migrating to the thymus for maturation.
    • B-cell precursors are derived from bone marrow stem cells.
    • T-cell maturation involves acquiring a unique T cell receptor (TCR).
    • B-cell maturation involves rearranging immunoglobulin genes to create a unique B-cell receptor (BCR).

    T-Cell Subtypes

    • CD4+ T-cells are helper cells crucial for coordinating immune responses.
      • Th1 cells help fight intracellular pathogens (e.g., viruses, bacteria).
      • Th2 cells help fight extracellular parasites and activate B cells.
      • Th17 cells help fight extracellular bacteria and fungi.
    • CD8+ T-cells (cytotoxic T-lymphocytes or CTLs) directly kill infected or cancerous cells.

    Lymphocyte Differentiation

    • Naïve lymphocytes are unactivated, meaning they haven't encountered their specific antigen yet.
    • Plasma cells are fully differentiated B cells that secrete antibodies.
    • ​Memory cells are long-lived lymphocytes that can rapidly respond to a reinfection with a specific antigen.

    Clonal Selection

    • Positive selection ensures that only T and B cells that have functional receptors survive.
    • Negative selection eliminates lymphocytes that strongly bind self-antigens, preventing autoimmune reactions.

    Antibody Structure

    • Antibodies (immunoglobulins) are glycoproteins composed of light (L) and heavy (H) chains.
    • The Fab region binds antigens.
    • The Fc region mediates effector functions like antibody-dependent cellular cytotoxicity or complement activation.

    Functions of Lymphocytes

    • T-cells: Host defense against infection, tumor rejection, immune regulation, allergy/hypersensitivity, autoimmunity, transplant graft rejection.
    • B-cells: Host defense against infection, allergy/hypersensitivity, autoimmunity.

    Natural Killer (NK) Cells

    • Lack T-cell receptors and surface immunoglobulins.
    • Recognize and kill virus-infected cells and cancer cells through nonspecific mechanisms.
    • Produce gamma interferon to activate macrophages.

    Lymphocytes: T and B Cells

    • Lymphocytes are a type of white blood cell crucial for the immune system. They defend against disease and infection.
    • T lymphocytes (T cells) are responsible for coordinating the immune response and directly attacking infected cells and tumor cells.
    • B lymphocytes (B cells) produce antibodies, proteins that target viruses, bacteria, and other foreign invaders.

    Origin and Maturation of T Cells

    • T-cell precursors originate from common lymphoid progenitors in the bone marrow.
    • They migrate to the thymus for maturation, where they acquire unique T-cell receptors (TCRs).
    • Mature, naïve T cells exit the thymus, having never encountered foreign antigens.

    Development of Naïve Lymphocytes

    • Naïve lymphocytes are those that have not yet interacted with a specific antigen.
    • Upon encountering an antigen, naïve lymphocytes differentiate into either effector cells or memory cells.

    T-Cell Receptor Structure

    • TCRs have two chains (α and β) that bind to peptide-MHC complexes.
    • The variable region of the TCR recognizes antigen, while the constant region connects the α and β chains.
    • CD3 proteins are essential for signaling TCR recognition from the cell surface to the inside of the cell.
    • CD4 and CD8 coreceptors bind MHC molecules and amplify TCR signaling.

    Thymic Selection

    • Positive Selection: T-cell precursors in the thymus cortex bind to MHC molecules expressed by specialized thymus cells. Only those that bind are allowed to survive.
    • Negative Selection: In the thymus medulla, T cells making strong contact with self-antigens are eliminated by apoptosis. This ensures tolerance to self.

    T-Cell Subtypes

    • CD4+ T Cells: These cells assist other immune cells, and they can be further classified into effector/helper (Teff/Th), follicular helper (Tfh), and regulatory T (Treg) cells.
      • Teff/Th: Coordinate immune responses in inflamed tissues.
      • Tfh: Help B cells in the B-cell follicle of lymphoid organs.
      • Treg: Suppress inflammation.
      • Th1, Th2, and Th17: Specific effector T-cell subsets with distinct functions.
    • CD8+ T Cells (Cytotoxic T Lymphocytes, CTLs): Recognize and kill cells displaying foreign proteins on their surface, like virus-infected or cancerous cells.

    Origin and Maturation of B Cells

    • B-cell precursors arise in the fetal liver, but after birth, they primarily develop in the bone marrow.
    • The thymus is not involved in B-cell maturation.
    • B-cell maturation has two phases:
      • Antigen-independent phase: B cells rearrange their immunoglobulin genes to create unique antigen receptors (B-cell receptors, BCRs).
      • Antigen-dependent phase: Mature B cells interact with antigens.

    Immunoglobulin (Ig) Structure

    • Antibodies (immunoglobulins) are Y-shaped glycoproteins composed of two identical heavy (H) chains and two identical light (L) chains.
    • The antigen-binding fragment (Fab) contains variable (VL and VH) and constant (CL and CH1) regions.
    • The constant fragment (Fc) consists of the remaining constant regions of the H chains (CH2, CH3, etc.).

    Plasma and Memory Cells

    • Plasma Cells: Produce antibodies in response to antigens. They are short-lived effector cells.
    • Memory Cells: Long-lived cells generated from antigen-stimulated lymphocytes. They can rapidly respond to a reinfection with the same antigen.

    Clonal Selection

    • Positive Selection: B-cell precursors with functional BCRs are allowed to survive.
    • Negative Selection: B cells that strongly bind self-antigens in the bone marrow are removed through apoptosis. This prevents autoimmune reactions.

    NK Cells (Natural Killer Cells)

    • NK cells are part of the innate immune system, not lymphocytes.
    • They are cytotoxic, meaning they can kill cells, but they do not need prior exposure to antigens.
    • NK cells play a role in recognizing and destroying infected or cancerous cells.

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    T and B Lymphocytes PDF

    Description

    Explore the process of lymphocyte differentiation into effector and memory cells, triggered by antigen recognition. Learn about the structure of T-cell receptors, including their two-chain composition and essential components for signal transduction. This quiz also covers the process of thymic selection in T-cell development.

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