In-Depth Notes on Immune Response

Antigen Specific Immune Response

Overview of Immune Response

  • Types of Immune Response:
    • Humoral Response:
    • Targets extracellular antigens.
    • Example: Bacterial and fungal infections.
    • Cell-Mediated Response:
    • Targets infected or irregular cells.
    • Example: Sickle cell, cancer cells, mutated cells.

Antigens

  • Definition: Substances that react with antibodies or antigen receptors on white blood cells (WBCs).
  • Characteristics:
    • Foreign material (e.g., pollen).
    • High molecular weight.
    • Complex in structure.
  • Chemical Nature:
    • Primarily proteins; some polysaccharides.
  • Epitopes:
    • Small regions on antigens recognized by the immune system.
    • Example: E. coli presents various epitopes.

Cell Receptors

  • Antigen-Presenting Cells (APCs):
    • Function: Engulf and present antigens to initiate the adaptive immune response.
    • Types:
    • Macrophages
    • Dendritic Cells
  • MHC Molecules:
    • Involved in distinguishing self from non-self.
    • MHC Class I:
    • For endogenous antigens (from within the body).
    • Found on all nucleated cells.
    • MHC Class II:
    • For exogenous antigens (found outside the body).
    • Present on APCs only.

B-Lymphocytes (Humoral Response)

  • B Cell Receptors: Specialized receptors on B cells.
  • Function: Produce antibodies and mature into plasma cells (effector cells).

T-Lymphocytes (Cell-Mediated Response)

  • T Helper Cells (T-4):
    • Contain CD4 receptors.
    • Interact with MHC Class II to release cytokines, which stimulate B and T cell proliferation and differentiation.
  • Cytotoxic T Cells (T-8):
    • Contain CD8 receptors.
    • Responsible for killing infected or altered cells via apoptosis and releasing cytokines.
  • Natural Killer Cells:
    • Lacks specialized receptors.
    • Indiscriminately kills all contacted cells, healthy or otherwise.

Lymphatic System

  1. Primary Organs:
    • Produce or mature lymphocytes.
    • Bone Marrow: Produces all blood cells and matures B cells.
    • Thymus: Matures T cells.
  2. Vessels: Conduct lymph cells.
  3. Secondary Organs: Storage for lymphocytes.

Steps of the Adaptive Immune Response

  1. Encounter of antigen.
  2. Clonal selection of lymphocytes.
  3. Clonal expansion (multiplication of selected lymphocytes).
  4. Differentiation into effector cells and memory cells.
  5. Formation of memory cells for future immunity.

Antibodies

  • Structure: Y-shaped molecule composed of four glycoprotein chains.
    • Fab region: Antigen-binding part.
    • Fc region: Constant region that attaches to self-cells.
  • Classes of Antibodies:
    • IgG: Most common antibody; crosses the placenta.
    • IgM: First antibody in response (can switch to any other class).
    • Pentameric structure (10 antigen-binding sites).
    • IgA: Found in mucosal secretions; prevents pathogen attachment (dimeric structure).
    • IgD: Serves as a B cell receptor.
    • IgE: Associated with mast cells; mediates allergic responses and defends against parasites.

Functions of Antibodies

  • Neutralization: Prevents pathogen-host interaction by coating the antigen.
  • Unmobilization: Attaches to flagella impairing motility.
  • Agglutination: Clumping of antigens together for easier clearance.
  • Opsonization: Enhances phagocytosis efficiency by marking pathogens.
  • Antibody-Dependent Cellular Cytotoxicity: Natural killer cells recognize and kill marked targets.

Types of Immunity

  1. Naturally Acquired Immunity:
    • Active: Body produces a memory response after an infection.
    • Passive: Transfer of antibodies via breast milk.
  2. Artificially Acquired Immunity:
    • Active: Through vaccinations.
    • Passive: Administration of antitoxins or monoclonal antibodies.