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Questions and Answers
What distinguishes adaptive immunity from innate immunity?
What distinguishes adaptive immunity from innate immunity?
Which cells primarily produce antibodies in the adaptive immune response?
Which cells primarily produce antibodies in the adaptive immune response?
What is the main role of CD8+ cytotoxic T cells in adaptive immunity?
What is the main role of CD8+ cytotoxic T cells in adaptive immunity?
What function do memory B cells serve in adaptive immunity?
What function do memory B cells serve in adaptive immunity?
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Which type of immunity primarily involves the activation of B cells?
Which type of immunity primarily involves the activation of B cells?
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How do dendritic cells contribute to adaptive immunity?
How do dendritic cells contribute to adaptive immunity?
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What is the primary mechanism through which antibodies enhance immune defense?
What is the primary mechanism through which antibodies enhance immune defense?
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What type of cells serve as antigen-presenting cells (APCs)?
What type of cells serve as antigen-presenting cells (APCs)?
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What is the role of cytokines IL-4 and IL-5 in B cell activation?
What is the role of cytokines IL-4 and IL-5 in B cell activation?
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Which antibody class is primarily produced during the primary immune response?
Which antibody class is primarily produced during the primary immune response?
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What happens to B cells in the dark zone of the germinal centers?
What happens to B cells in the dark zone of the germinal centers?
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How does the secondary immune response differ from the primary immune response?
How does the secondary immune response differ from the primary immune response?
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What is the process of Ig class switching in B cells?
What is the process of Ig class switching in B cells?
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What primarily characterizes memory B cells?
What primarily characterizes memory B cells?
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What is a significant benefit of herd immunity?
What is a significant benefit of herd immunity?
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During which phase do naive B cells primarily encounter their specific antigen?
During which phase do naive B cells primarily encounter their specific antigen?
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What is the main function of somatic hypermutation in B cells?
What is the main function of somatic hypermutation in B cells?
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Which type of T cell is primarily involved in activating B cells?
Which type of T cell is primarily involved in activating B cells?
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What characteristic distinguishes immunogens from antigens?
What characteristic distinguishes immunogens from antigens?
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Which cells primarily present exogenous antigens to T cells?
Which cells primarily present exogenous antigens to T cells?
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What type of antigens do MHC class I molecules present?
What type of antigens do MHC class I molecules present?
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What is a primary role of CD4+ T cells once activated?
What is a primary role of CD4+ T cells once activated?
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How does MHC polymorphism contribute to the immune system?
How does MHC polymorphism contribute to the immune system?
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During which stage do T cells express both CD4 and CD8 markers?
During which stage do T cells express both CD4 and CD8 markers?
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What process eliminates T cells that strongly recognize self-antigens?
What process eliminates T cells that strongly recognize self-antigens?
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Which type of antigen processing involves presentation of intracellular proteins?
Which type of antigen processing involves presentation of intracellular proteins?
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Which cells are primarily involved in the positive selection of T cells?
Which cells are primarily involved in the positive selection of T cells?
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Which type of antigens are recognized by CD8+ T cells?
Which type of antigens are recognized by CD8+ T cells?
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What defines a hapten?
What defines a hapten?
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What role do naive T cells play in the immune system?
What role do naive T cells play in the immune system?
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Which cells can CD8+ T cells be activated directly by?
Which cells can CD8+ T cells be activated directly by?
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What kind of antigens do CD4+ T cells recognize through MHC class II molecules?
What kind of antigens do CD4+ T cells recognize through MHC class II molecules?
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What is the primary role of IL-2 in T cell activation?
What is the primary role of IL-2 in T cell activation?
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Which interaction enhances CD4+ T cell activation and the immune response?
Which interaction enhances CD4+ T cell activation and the immune response?
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Which pathway requires CD4+ helper T cells for B cell activation?
Which pathway requires CD4+ helper T cells for B cell activation?
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What kind of antibodies are primarily produced through the T Cell-Independent Pathway?
What kind of antibodies are primarily produced through the T Cell-Independent Pathway?
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What is the primary function of the crystallizable fragment (Fc) region of an antibody?
What is the primary function of the crystallizable fragment (Fc) region of an antibody?
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What process increases the diversity of antibodies after antigen activation?
What process increases the diversity of antibodies after antigen activation?
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Which type of T cells directly help to activate CD8+ T cells?
Which type of T cells directly help to activate CD8+ T cells?
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What happens when activated B cells differentiate?
What happens when activated B cells differentiate?
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How do B cells recognize antigens in the T Cell-Independent Pathway?
How do B cells recognize antigens in the T Cell-Independent Pathway?
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What is one main characteristic of IgM antibodies produced in T Cell-Independent activation?
What is one main characteristic of IgM antibodies produced in T Cell-Independent activation?
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What is the role of CD40 in T cell activation?
What is the role of CD40 in T cell activation?
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Study Notes
Adaptive Immunity Overview
- Adaptive immunity is a specific defense system against pathogens, developing over time and targeting unique antigens.
- Unlike innate immunity, adaptive immunity is slower, requiring days to weeks for full activation.
- Adaptive immunity is highly specific, targeting unique antigens, and exhibits immunological memory, becoming more efficient upon repeated exposures.
Types of Adaptive Immunity
- Antibody-mediated (Humoral) Immunity: Involves B cells producing antibodies, proteins that neutralize pathogens and toxins, enhance phagocytosis, and provide targeted defense at mucosal surfaces.
- Cell-mediated Immunity: Involves the activation of cytotoxic T cells (CD8+ T cells), which directly kill infected cells or bacteria through lysis.
Cells of Adaptive Immunity
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T Lymphocytes (T Cells): Crucial in cell-mediated immunity.
- CD4+ Helper T Cells: Orchestrate the immune response, activating other immune cells (B cells, macrophages, CD8+ T cells).
- CD8+ Cytotoxic T Cells (CTLs): Directly kill infected cells or bacteria.
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B Lymphocytes (B Cells): Responsible for antibody production.
- Plasma Cells: Activated B cells secreting large amounts of antibodies.
- Memory B Cells: Long-lived cells providing immunological memory, enabling a faster, stronger response to subsequent exposures.
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Antigen-Presenting Cells (APCs): Bridge innate and adaptive immunity by capturing, processing, and presenting antigens to T cells.
- Dendritic Cells: Most effective APCs, linking these two systems. They express pathogen-recognition receptors (PRRs) like TLRs to detect various pathogens. Upon activation, they migrate to lymph nodes and present antigens to T cells, initiating the adaptive response.
- Macrophages: Can also present antigens but are less efficient at migrating to lymph nodes.
Antigens: Targets of the System
- Antigen: Any molecule recognized by the immune system, typically from microbes, transformed cells (e.g., tumors), or self-cells.
- Epitope: The specific region of an antigen recognized by antibodies or T cell receptors.
- Haptens: Small molecules that are not immunogenic on their own; become immunogenic when bound to a larger carrier molecule.
- Immunogen: A subset of antigens that can directly trigger an immune response. All immunogens are antigens, but not all antigens are immunogens.
Antigen Processing and Presentation
- Adaptive immunity relies on antigen presentation by APCs to T cells.
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MHC Class I Pathway: Presents endogenous antigens (produced inside the cell, e.g., viral proteins, intracellular bacteria).
- Involves most nucleated cells.
- Intracellular antigens are broken down, transported, and loaded onto MHC class I molecules. The resulting complex travels to the cell surface to be presented to CD8+ T cells, activating them into CTLs.
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MHC Class II Pathway: Presents exogenous antigens (from outside the cell, e.g., bacterial proteins, phagocytosed pathogens).
- Primarily occurs in professional APCs (dendritic cells, macrophages, B cells)
- Exogenous antigens are taken up, processed, and bound to MHC class II molecules. The complex is displayed on the cell surface for presentation to CD4+ T cells, activating them.
MHC Polymorphism
- MHC genes are highly polymorphic (many versions or alleles within a population), crucial for immune system diversity.
- Inherited unique combinations of MHC alleles ensure individual variations in antigen recognition.
- While providing broad recognition, individual MHC molecules cannot present all antigens, explaining why some individuals are more susceptible to certain infections.
T Cell Development and Selection
- T cells originate in the bone marrow and mature in the thymus.
- In the thymus, T cells differentiate and mature in thymus lobules.
- T cell receptor (TCR) diversity: Variable region genes of the TCR undergo rearrangement, creating a vast repertoire of antigen-recognizing TCRs.
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T cell selection: T cells interact with MHC molecules on thymic cells.
- Positive selection: T cells that recognize self-MHC are selected for survival.
- Negative selection: T cells that strongly bind self-antigens presented on MHC are eliminated to prevent autoimmunity.
- Maturation concludes with T cells expressing either CD4+ or CD8+ and their specific functions.
- Naive T cells: Mature T cells not yet exposed to their specific antigen, circulating in the blood and lymphatic system.
T Cell Activation
- Naive T cells become activated when their TCR recognizes a specific antigen presented by MHC molecules on APCs.
- CD8+ T cell activation: Activated by MHC class I presenting intracellular antigens.
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CD4+ T cell activation: Activated by MHC class II presenting extracellular antigens.
- CD40-CD40 ligand interaction enhances the response to antigen presentation.
Role of IL-2
- Activated T cells (CD4+ and CD8+) produce IL-2 (cytokine promoting T cell proliferation)
- IL-2 receptors are upregulated on activated T cells.
- IL-2 acts autocrine and paracrine to stimulate T cell division and differentiation, leading to clonal expansion and a more robust immune response.
CD4+ Helper T Cell Functions
- B cell activation: CD4+ helper T cells provide crucial signals for B cell activation, proliferation, and differentiation into antibody-producing plasma cells.
- Macrophage activation: CD4+ T cells secrete cytokines activating macrophages, enhancing their pathogen-killing ability and inflammatory response.
- CD8+ T cell activation: Direct activation help to CD8+ T cells.
B Cell Activation and the Antibody Response
- B cell receptor (BCR): B cells express BCRs specific for particular antigens.
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Two Activation Pathways:
- T cell-dependent: Requires CD4+ helper T cells, producing highly specific IgG antibodies and memory B cells.
- T cell-independent: Doesn't require T cells; triggered by certain antigens (e.g., LPS), producing mainly IgM antibodies with limited specificity, without memory B cells.
Immunoglobulins (Antibodies)
- Antibodies (Ig) are crucial components of the adaptive immune system. They perform various functions: neutralizing pathogens, opsonizing microbes for phagocytosis, activating complement, and preventing microbial attachment to mucosal surfaces.
- Their structure consists of light and heavy chains, variable (antigen-binding) regions, and constant (complement binding) regions.
- Antibody diversity is achieved through gene rearrangement and somatic hypermutation.
Antibody Diversity Generation
- Gene rearrangement: Reshuffling of variable gene regions of light chains.
- Somatic hypermutation: Point mutations in variable regions leading to greater diversity and enhanced antigen affinity.
Consequences of T Cell Help for B Cell Activation
- Clonal expansion: Cytokines (IL-4, IL-5) drive B cell proliferation into clones.
- Isotype switching: Generates different antibody classes (from IgM to IgG, IgA, or IgE) switching based on T helper cell help.
- Somatic hypermutation: Introducing point mutations in variable regions for further diversity and affinity maturation.
B Cell Memory and Long-Term Immunity
- Memory B cells: Long-lived cells generated during T cell-dependent B cell activation.
- Secondary response: Upon re-exposure to the same antigen, memory B cells induce a rapid, stronger response producing primarily high-affinity IgG antibodies.
Phases of B Cell Differentiation
- Phase 1: Initial activation and migration of naive B cells.
- Phase 2: Germinal center formation and differentiation of activated B cells into long-lived plasma cells or memory B cells.
Secondary Immune Response
- Faster and stronger response upon second antigen exposure due to memory B and T cells.
- Peak antibody response occurs faster, producing predominantly high-affinity IgG antibodies.
Vaccination and Immunological Memory
- Vaccines mimic natural infection, inducing a primary immune response leading to memory B and T cell formation.
- Subsequent encounters with the pathogen lead to a robust secondary response.
Summary: Timeline of Immune Response
- Primary response (first exposure): Peak antibody levels around 7 days, predominantly IgM, some IgG.
- Secondary response (second exposure): Peak antibody response within 3 days, significantly higher IgG production
Herd Immunity
- Widespread vaccination generates herd immunity, limiting pathogen transmission and protecting even unvaccinated individuals.
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Description
Explore the intricate mechanisms of adaptive immunity, a critical defense system against pathogens. This quiz covers the distinctions between antibody-mediated and cell-mediated immunity, as well as the role of various immune cells. Test your knowledge of how adaptive immunity develops and its implications in the immune response.
