Immunology: Antibodies and B Cell Receptors
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Questions and Answers

Which of the following statements about antibodies is true?

  • The constant region of antibodies has a high level of variability.
  • Antibodies can neutralize pathogens by blocking their adherence. (correct)
  • All antibodies can recognize any epitope on a pathogen.
  • Antibodies directly kill pathogens by lysis.

    • During the maturation of B cells, which characteristic is retained among the clones generated?

  • Diversity of antigen recognition.
  • Identical antigen specificity. (correct)
  • Capability to respond to all antigens.
  • Variability in surface membrane proteins.

    • What is the primary role of Memory B cells?

  • To produce a wide variety of antibodies.
  • To generate new antibodies against foreign pathogens.
  • To kill infected cells directly.
  • To facilitate rapid response upon re-infection. (correct)

    • Which activity is NOT a function of antibodies?

    <p>Directly engulfing pathogens.</p> Signup and view all the answers

    What distinguishes T cell receptors (TCRs) from antibodies in recognizing antigens?

    <p>TCRs recognize epitopes bound to MHC proteins.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of MHC molecules?

    <p>MHC molecules are the same in all individuals.</p> Signup and view all the answers

    Which is a characteristic feature of IgM antibodies?

    <p>They are produced first during a primary immune response.</p> Signup and view all the answers

    Which property of acquired immunity allows it to identify and differentiate between similar but distinct pathogens?

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

    What is the primary effect of opsonization by antibodies?

    <p>Enhancement of phagocytosis.</p> Signup and view all the answers

    What fundamental property of acquired immunity ensures that the immune system can adapt to a vast number of pathogens?

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

    What is the primary function of B cells in the adaptive immune response?

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

    Which step in the immune response occurs after the activation and proliferation of B cells?

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

    Why is memory crucial in adaptive immunity?

    <p>It allows for quicker responses upon re-exposure to the same pathogen</p> Signup and view all the answers

    What is the primary role of T-helper cells in the antibody response?

    <p>Activate B cells for antibody production</p> Signup and view all the answers

    Which statement accurately describes the specificity of antibodies?

    <p>A single B cell can only produce one type of antibody specificity</p> Signup and view all the answers

    What is a characteristic feature of the memory capability of adaptive immunity?

    <p>Memory B cells can recognize the same pathogen in future exposures</p> Signup and view all the answers

    What are the main functions of antibodies?

    <p>Bind specifically to pathogens and neutralize or mark them for removal</p> Signup and view all the answers

    In relation to B cell maturation, what is clonal selection?

    <p>The selection and proliferation of B cells that specifically recognize an antigen</p> Signup and view all the answers

    Which of the following accurately describes the antibody secretion process?

    <p>Plasma cells are responsible for the secretion of antibodies after activation</p> Signup and view all the answers

    How do antibodies facilitate the clearance of pathogens?

    <p>By binding and neutralizing toxins and pathogens for removal</p> Signup and view all the answers

    What primarily distinguishes cell-mediated immunity from antibody-mediated immunity?

    <p>Cell-mediated immunity involves T cells that directly kill infected or altered self-cells</p> Signup and view all the answers

    In terms of the types of antigens, which of the following best describes the adaptive immune response?

    <p>It encompasses responses against both extracellular and intracellular antigens</p> Signup and view all the answers

    What is the primary role of MHC class II molecules in the immune response?

    <p>To present exogenous antigens to T helper cells</p> Signup and view all the answers

    How does MHC class I differ from MHC class II in terms of peptide processing?

    <p>MHC class I presents peptides from intracellular pathogens only</p> Signup and view all the answers

    Which statement accurately describes cytotoxic T lymphocytes (CTL)?

    <p>They recognize antigens presented on MHC class I</p> Signup and view all the answers

    What type of cells primarily utilize MHC class II molecules?

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

    What is the source of peptides presented by MHC class II molecules?

    <p>Digestion of exogenous antigens taken up by phagocytosis</p> Signup and view all the answers

    What is one of the major functions of T helper cells in the adaptive immune response?

    <p>Provide help for B cells to produce antibodies</p> Signup and view all the answers

    How does genetic variation in TCR and MHC contribute to the immune response?

    <p>It enhances the ability to recognize a wide array of antigens</p> Signup and view all the answers

    Why is it important for MHC molecules to present peptides on the cell surface?

    <p>To enable recognition by T cells</p> Signup and view all the answers

    What determines whether a peptide associates with MHC class I or MHC class II?

    <p>The source of the antigen and cellular processing pathways</p> Signup and view all the answers

    What type of antigens do cytotoxic T cells primarily recognize?

    <p>Endogenous antigens presented on MHC class I</p> Signup and view all the answers

    What distinguishes MHC-I from MHC-II in terms of peptide sources?

    <p>MHC-I presents endogenous antigens, whereas MHC-II presents exogenous antigens.</p> Signup and view all the answers

    What is the primary function of MHC-I molecules?

    <p>To indicate intracellular infections and signal for cell death.</p> Signup and view all the answers

    Which statement correctly describes the processing site for MHC-II?

    <p>MHC-II peptides are processed in the phagolysosome.</p> Signup and view all the answers

    What happens to self-peptides presented by MHC during a non-infected state?

    <p>They do not activate the immune response.</p> Signup and view all the answers

    How do macrophages respond upon binding with MHC-II?

    <p>They activate and become more efficient phagocytes.</p> Signup and view all the answers

    What type of antigens does MHC-II primarily present to T cells?

    <p>Extracellular antigens picked up by phagocytosis.</p> Signup and view all the answers

    Which cell types are primarily identified as MHC-II presenting cells?

    <p>Dendritic cells, macrophages, and B cells.</p> Signup and view all the answers

    What is the consequence of a foreign or altered-self peptide being presented on MHC-I?

    <p>It signals Killer T cells to destroy the presenting cell.</p> Signup and view all the answers

    What role does the T cell receptor (TCR) play in binding to MHC?

    <p>It mediates interaction between T cells and antigen-presenting cells.</p> Signup and view all the answers

    When TCR binds to MHC-II, what is the typical outcome for the presenting cell?

    <p>It gets activated to present more antigens.</p> Signup and view all the answers

    Study Notes

    Antibodies (Abs)

    • Abs bind to a specific site on the surface of a pathogen called an epitope
    • A single pathogen can contain numerous epitopes
    • Abs usually recognize external epitopes on pathogens
    • Abs can recognize different epitopes on the same pathogen
    • Abs have a constant region (Fc region) that binds receptors on immune cells
    • Abs have variable regions that are specific to each antigen
    • Abs can bind to antigens on bacteria, viruses, toxins or fungal cells.

    B Cell Receptor (BCR)

    • B cells express BCRs on their surface
    • BCRs are membrane-bound immunoglobulins
    • BCRs are responsible for recognizing and binding to antigens

    B Cell Maturation & Clonal Selection

    • B cells mature in the bone marrow
    • Naive B cells are immature and have not yet encountered an antigen
    • When a B cell encounters an antigen that matches its BCR, it activates and proliferates
    • All of the progeny of the activated B cell are clones of the original cell and have identical antigen specificity
    • Some clones become plasma cells that secrete antibodies
    • Other clones become memory B cells that remain in the body and can rapidly respond to a reinfection

    Biological Activities of Abs

    • Opsonization: Abs coat antigens, making them more attractive for phagocytosis by macrophages or neutrophils
    • Activation of Complement: Abs, when bound to bacteria or other pathogens, can activate the complement cascade.
    • Complement activation leads to:
      • lysis of bacterial cells
      • opsonization
      • inflammation
    • Antibody-dependent cell-mediated cytotoxicity (ADCC): some white blood cells have Fc receptors that bind to the Fc region of antibodies, facilitating the killing of target cells.
    • Neutralization: Abs can bind to the surface of pathogens to block adhesion or can bind toxins and block their activity
    • Agglutination: Abs can bind to multiple antigens at one time, clumping them together and preventing their spread.

    5 Ab Isotypes

    • IgM: The first antibody produced in a primary immune response
      • The membrane-bound form of IgM serves as the B cell receptor
      • The secreted form of IgM is a pentamer, which allows it to bind many antigens simultaneously
      • It is highly efficient at activating complement.
    • IgD: Found on the surface of mature B cells. Its function is still unknown.
    • IgE: Mediates hypersensitivity reactions such as hay fever, asthma, hives, and anaphylactic shock
      • Binds to Fc receptors on mast cells and basophils.
    • IgG: The most abundant antibody in the serum.
      • Can cross the placenta and provides immunity to the fetus
      • Activates complement and binds Fc receptors on phagocytic cells
    • IgA: The predominant antibody in external secretions such as breast milk, saliva, tears, and mucus
      • Prevents pathogens from attaching to mucosal surfaces
      • Protects newborns during their first month of life

    T Lymphocytes (T Cells)

    • Produced in the bone marrow and mature in the thymus
    • Circulate in the lymph and blood and migrate to the lymph nodes, spleen, and Peyer's patches
    • T cells have highly specific T cell receptors (TCRs) on their plasma membranes that recognize antigens
    • TCRs do not directly recognize epitopes but only recognize epitopes associated with a major histocompatibility complex (MHC) protein
    • T cells primarily act against body cells that harbor intracellular pathogens.

    T Cell Types

    • Helper T cells (TH cells): Send signals to activate other parts of the immune response
    • Cytotoxic T cells (TC cells or CTLs): Directly kill altered-self cells (e.g., virus-infected cells, tumor cells)
    • Regulatory T cells (TR cells): Repress adaptive immune responses

    MHC (Major Histocompatibility Complex)

    • A collection of genes on chromosome 6
    • MHC genes are highly polymorphic, meaning that there is a lot of variation between individuals.
    • The only people who share the same MHC are identical twins.
    • Acts as a "self" marker on the surface of cells
    • Two classes of MHC:
      • MHC Class I: Found on nearly all cells in the body
      • MHC Class II: Found on "professional" antigen-presenting cells (APCs) such as: macrophages, dendritic cells, and B cells

    MHC and Antigen Processing

    • MHC molecules bind peptides derived from proteins and display them on the cell surface
    • Peptides can be either "self" or "foreign"
    • MHC-peptide complex formation is important because it allows T cells to recognize the presence of foreign antigens inside or outside the cell
    • MHC molecules are like "highway warning signs" to T cells.
    • During normal conditions (absence of infection), MHCs display self peptides.
    • T cells recognize these self peptides and do not attack the cell
    • Under infection, MHCs can display foreign peptides.
      • T cells recognize these foreign peptides, which acts as a "danger signal"
    • MHC Class I presents endogenous antigens
    • The immune response is focused on dealing with intracellular infections such as viruses.
    • MHC Class II presents exogenous antigens
      • The immune response is focused on dealing with extracellular infections such as bacteria.

    MHC Class I: Presentation of Endogenous Peptides

    • MHC Class I presents peptides that are derived from proteins produced inside the cell:
    • Inside the cell, a viral protein is broken down and loaded onto MHC Class I
    • Viral peptide loaded on MHC Class I signals that there is an intracellular infection
    • The cell is displayed on the cell surface
    • The infected cell is killed by cytotoxic T lymphocytes

    MHC Class II: Presentation of Exogenous Peptides

    • MHC Class II presents peptides that are derived from proteins that have been taken into the cell:
    • A phagocytic cell takes up a bacteria or toxin and breaks it down in a phagolysosome
    • Peptides from the breakdown are loaded onto MHC Class II.
    • MHC II/peptide complex is displayed on the cell surface
    • Helper T Cells that recognize MHC II/peptide complex become activated.

    The Adaptive Immune Response: B Cells & T Cells

    • Cell-mediated immunity: T cell-mediated. Focuses on intracellular pathogens such as viruses (killing cells) or tumors.
    • Antibody response: B cell-mediated. Focuses on extracellular pathogens such as bacteria or free viruses.
    • Both cell-mediated and antibody responses require Helper T cells (TH cells)

    B Cell & Ab Facts

    • Each B cell can only make one specificity of antibody
    • Another name for antibody is immunoglobulin (Ig)
    • Antibodies are found:
      • stuck to the surface of a B cell (B cell receptor: BCR)
      • secreted - circulating in the blood and lymph or patrolling mucosal surfaces

    Two Main Functions of Abs are Structurally Separated

    • Binding specifically to pathogens: Accomplished by the variable regions of the antibody.
    • Binding to MHC I and MHC II: This is accomplished by the constant regions of the antibody which then allow the immune cells to recognize and interact with the antibodies.

    Two Kinds of T Cells Match Two Kinds of MHC

    • TC (CTL): Cytotoxic T cells directly kill infected cells. They recognize antigen presented on MHC Class I
    • TH: Helper T cells are important for the initiation of the adaptive immune response. They recognize antigen presented on MHC Class II

    TCR/MHC-Peptide Complex Formation

    • The binding between T cells and antigen-MHC complex is extremely specific:
    • There is a lot of genetic variation in both the TCR and MHC
    • This variation ensures that the immune system can recognize and bind to a wide variety of antigens.

    Two Pathways for antigen processing

    • MHC Class I:
    • Endogenous antigens are processed inside the cell
    • The pathway is important to eliminate pathogens that are already inside cells.
    • MHC Class II:
    • Exogenous antigens are taken up and processed by phagocytosis.
    • The pathway is important to deal with pathogens that are outside the cell.

    Immune Response Summary

    • Immune responses are determined by the nature of the pathogen and the mechanism by which antigens are processed.
    • MHCs are essential for the adaptive immune response because they inform the T cells about the state of the host's cells.

    Overview of Body Defenses

    • Non-Specific Resistance (Innate Immunity): The first and second lines of defense are non-specific. They act against a wide range of pathogens.
      • First line of defense:
        • Skin
        • Mucous membranes and their secretions
        • Normal flora (microorganisms that normally live on the body)
        • Antimicrobial substances (e.g., lysozyme in tears, skin oil)
      • Second line of defense:
        • Phagocytosis: cells that ingest bacteria or other pathogens
        • Inflammation
        • Fever
        • Extracellular killing by antimicrobial substances
    • Specific Resistance (Adaptive Immunity): The third line of defense
      • Third line of defense:
      • Specialized Lymphocytes:
        • T cells: (Helper & Killer cells)
        • B cells: (Antibodies)

    Adaptive Immunity

    • The adaptive immune response is specific. Each cell is specialized to recognize and respond to only one specific antigen.
    • The adaptive immune response is acquired because it is developed upon exposure to a pathogen.
    • The adaptive immune response takes time to develop (about 5-7 days)
    • The adaptive immune response is mediated by B cells (make antibodies) and T cells (either help other immune cells or are killer cells).

    Why does adaptive immunity take so long?

    • The body has to generate a specific response, which involves expanding the numbers of T cells that specifically recognize a particular pathogen.

    Fundamental Properties of Acquired Immunity

    • Learning: The immune system learns to distinguish between self and non-self.
    • Specificity: The immune system can discriminate between subtle differences in antigens (e.g., 1-2 amino acid changes).
    • Diversity: The immune system must have a broad range of specificities to be able to recognize the diverse array of pathogens that a person encounters.
      • There are approximately 100,000,000,000 different Ab specificities and 10,000,000,000,000 different T cell specificities!
    • Memory: The immune system remembers pathogens that it has encountered before and can mount a faster and more effective response if the same pathogen is encountered again.

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    Description

    This quiz covers essential concepts in immunology regarding antibodies (Abs) and B cell receptors (BCRs). Explore topics such as the binding mechanics of Abs, the structure of BCRs, and the maturation and activation of B cells. Test your understanding of these critical immune components and their roles in pathogen recognition.

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