Lymphocyte Differentiation and T-cell Structure

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

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.

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.