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.