KN

Lecture Notes: 16

Immune System Overview

  • Immune Response Types:
    • Innate Immunity:
    • First line of defense, constant presence from birth, immediate response.
    • Non-specific, identifies pathogen with some recognition.
    • Adaptive Immunity:
    • Specific to targets, slower to activate, creates memory cells after contact with an antigen.

Key Components of Adaptive Immunity

  • Key Features:

    • Develops post-exposure to an antigen.
    • Involves complex and slow development.
    • Functions via a cross-regulated network of immune components.
  • Two Main Branches:

    • Humoral Immunity:
    • Involves B cells that produce antibodies.
    • Plasma cells are effector B cells that secrete antibodies.
    • Cell-Mediated Immunity:
    • Involves T cells, which recognize antigens indirectly and have roles in stimulating B cells and killing infected host cells.

Key Terminology

  • Antigen: Molecule stimulating an immune response; recognized by the adaptive immune system.
  • Epitope: Specific binding site on an antigen; multiple exist per antigen.
  • Hapten: Small molecule that can trigger an immune response only when attached to a larger carrier protein; does not activate B cells by itself.

Immunogenicity and Specificity

  • Immunogenicity: Measure of effectiveness of an antigen in eliciting an immune response. Stronger responses are generated by antigens that differ more from host molecules.

    • Example: stronger response to pathogens due to significant molecular dissimilarities.
  • Immune Specificity:

    • Antibodies produced are specific to a single antigen ("Lock and Key" model).

Antibody Structure and Function

  • General Structure:

    • Composed of glycoproteins with two heavy and two light chains, linked by disulfide bonds.
    • Fab region: Contains variable regions for antigen binding.
    • Fc region: Engages with host cell receptors and complements.
  • Five Classes of Antibodies:

    • IgA, IgD, IgE, IgG, IgM—each has unique functions and properties based on their heavy chain structures.

Mechanisms of Action of Antibodies

  1. Neutralization: Prevents toxins and viruses from binding to host cells.
  2. Opsonization: Marks pathogens for phagocytosis.
  3. Complement System Activation: Destroys microbes through lysis.
  4. Immobility: Prevents pathogens from adhering to cell surfaces.
  5. Cross-Linking: Aggregates pathogens for removal from the body.
  6. ADCC: Targets infected cells for destruction by natural killer cells.

B Cell Activation Processes

  • T-cell Independent Activation:

    • Occurs via capping, where large repeating epitopes bind to B cell receptors directly.
  • T-cell Dependent Mechanism:

    • Requires help from T-helper cells, leading to generation of plasma cells and memory B cells through clonal expansion.

Humoral Immune Response

Primary Response

  • Initial encounter with an antigen activates specific B cells leading to:
    • Clonal expansion into plasma cells (produce antibodies) and memory B cells (provide future protection).

Secondary Response

  • Faster and stronger response upon subsequent exposure to the same antigen due to memory B cells.

Summary of Adaptive Immune Response

  • Improves with exposure to pathogens.
  • Orchestrated by lymphocytes with characteristics of specificity, memory, and tolerance.