뒤로The Adaptive Immune System: Structure, Function, and Mechanisms
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The Adaptive Immune System
Overview of Adaptive Immunity
The adaptive (or acquired) immune system is a highly specialized defense mechanism that responds to specific pathogens through the activation of lymphocytes. Unlike innate immunity, adaptive immunity is characterized by specificity, diversity, memory, and self-tolerance. It is divided into two main branches: humoral immunity (mediated by B lymphocytes and antibodies) and cell-mediated immunity (mediated by T lymphocytes).
Specificity: Ability to target specific antigens.
Diversity: Capacity to recognize a vast array of antigens.
Memory: Enhanced response upon subsequent exposures to the same antigen.
Self-tolerance: Ability to distinguish self from non-self, preventing autoimmunity.
Antigens and Epitopes
Definition and Structure
Antigens are macromolecules (often proteins or polysaccharides) that elicit an immune response. Each antigen contains specific regions called epitopes (antigenic determinants), which are recognized by immune cells.
Epitope: The precise molecular structure within an antigen that is recognized by B or T cell receptors.
Types of Antigens: Exogenous (external pathogens), endogenous (intracellular pathogens), and autoantigens (self-molecules).
Lymphocyte Specificity: B and T Cell Receptors
B Cell Receptors (BCRs) and T Cell Receptors (TCRs)
B and T lymphocytes express unique antigen receptors that determine their specificity. BCRs are membrane-bound antibodies, while TCRs are specialized proteins that recognize antigen fragments presented by MHC molecules.
BCRs: Bind directly to free antigens; composed of variable and constant regions.
TCRs: Recognize processed antigens presented by MHC I or II molecules on cell surfaces.
Diversity of Lymphocyte Receptors
The immune system can generate between and distinct lymphocyte receptors through genetic recombination, allowing recognition of a vast array of antigens.
Mechanism: DNA recombination of gene segments creates unique receptors on each lymphocyte.
Clonal Selection, Proliferation, and Memory
Clonal Selection and Differentiation
Upon encountering their specific antigen, lymphocytes undergo clonal selection, proliferating and differentiating into effector and memory cells. Effector cells combat the current infection, while memory cells provide long-term immunity.
Clonal Selection: Activation of lymphocytes with receptors specific to the antigen.
Proliferation: Rapid multiplication of selected lymphocytes.
Differentiation: Formation of effector and memory cells.
Primary and Secondary Immune Responses
The primary immune response occurs upon first exposure to an antigen and is slower and less robust. The secondary response, triggered by subsequent exposures, is faster and produces higher antibody levels due to memory cells.
Primary Response: Initial activation and expansion of lymphocytes.
Secondary Response: Rapid and amplified response due to memory cells.
Self-Tolerance and Autoimmunity
Mechanisms of Self-Tolerance
Self-tolerance ensures that B and T cells do not attack the body's own tissues. Central tolerance occurs during lymphocyte development in the bone marrow (B cells) or thymus (T cells), where self-reactive cells are deleted. Failure of self-tolerance can lead to autoimmune diseases.
Central Tolerance: Deletion of self-reactive lymphocytes during development.
Autoimmunity: Immune response against self-antigens due to breakdown of tolerance.
Humoral Immunity: B Lymphocytes and Antibodies
Antibody Structure and Function
B lymphocytes mediate humoral immunity by producing antibodies. Upon activation, B cells differentiate into plasma cells (which secrete antibodies) and memory B cells. Antibodies bind to antigens and mark pathogens for destruction.
Plasma Cells: Short-lived cells that secrete large amounts of antibodies.
Memory B Cells: Long-lived cells that respond rapidly upon re-exposure to the antigen.
Antibody Classes and Their Functions
Class | Main Functions | Location |
|---|---|---|
IgM | Complement activation, neutralization, agglutination; primary response antibody | Blood, lymph |
IgG | Complement activation, neutralization, opsonization, oxidation, agglutination, ADCC; crosses placenta | Blood, extracellular fluid |
IgA | Neutralization, agglutination; secretory antibody in tears, saliva, mucus, breast milk | Mucosal surfaces, secretions |
IgE | Triggers antiparasitic and allergic responses | Bound to mast cells, basophils |
IgD | Function unclear; may act as B cell receptor | B cell surface |
Antibody Actions
Antibodies mediate several mechanisms to eliminate pathogens:
Neutralization: Antibodies block pathogen binding sites, preventing infection.
Opsonization: Antibodies coat pathogens, enhancing phagocytosis.
Oxidation: Antibodies facilitate the production of reactive oxygen species to kill pathogens.
Agglutination: Antibodies cause pathogens to clump together, facilitating clearance.
ADCC (Antibody-Dependent Cellular Cytotoxicity): Antibodies recruit natural killer cells to destroy large pathogens.
Cell-Mediated Immunity: T Lymphocytes
Types of T Cells and Their Functions
T lymphocytes are central to cell-mediated immunity. The two main types are helper T cells (TH) and cytotoxic T cells (TC). Helper T cells coordinate immune responses by secreting cytokines, while cytotoxic T cells kill virus-infected and abnormal cells.
Helper T Cells (CD4+): Recognize antigens presented by MHC II molecules; secrete cytokines to regulate immune responses.
Cytotoxic T Cells (CD8+): Recognize antigens presented by MHC I molecules; kill infected or abnormal cells by inducing apoptosis.
Major Histocompatibility Complex (MHC)
T cell receptors recognize antigens only when presented by MHC molecules:
MHC I: Present on all nucleated cells; present endogenous antigens to CD8+ cytotoxic T cells.
MHC II: Present on antigen-presenting cells (APCs); present exogenous antigens to CD4+ helper T cells.
Activation and Actions of T Cells
Helper T cells are activated when their TCR binds to antigen-MHC II complexes on APCs, leading to cytokine secretion. Cytotoxic T cells are activated by antigen-MHC I complexes and helper T cell-derived cytokines (e.g., IL-2), then kill target cells by releasing perforins and granzymes.
Cytokines: Regulatory proteins secreted by helper T cells to stimulate other immune cells.
Perforins and Granzymes: Molecules released by cytotoxic T cells to induce apoptosis in infected cells.
Integration of Immunity and Response to Pathogens
Immune Response to Different Pathogen Types
Bacteria: Combated by complement, phagocytosis, neutrophils, macrophages, helper T cells (Th1/Th2), and antibodies.
Viruses: Targeted by natural killer cells, interferons, helper T cells (Th1), cytotoxic T cells, and antibodies.
Fungi and Protozoa: Controlled by phagocytosis, dendritic cells, macrophages, helper T cells, and sometimes antibodies.
Helminths: Targeted by eosinophils, mast cells, helper T cells (Th2), and antibodies.
Additional info: The adaptive immune system works in concert with the innate immune system to provide comprehensive protection against a wide variety of pathogens, utilizing both cellular and humoral mechanisms.
