Adaptive Immunity - Comprehensive Notes

Chapter Overview

  • Adaptive Immune Response
    • Learned (acquired) immunity, high specificity for particular pathogens.
    • Has memory allowing for rapid response upon re-exposure.

Mechanisms of Adaptive Immunity

  • Types of Adaptive Immunity:
    • Humoral Immunity: Mediated by B cells.
    • Cellular (Cell-mediated) Immunity: Mediated by T cells.

Humoral Immunity

  • Targets extracellular antigens:
    • Pathogens: Bacteria, viruses in the bloodstream.
    • Toxins
  • Process:
    • B cells differentiate into plasma cells to produce antibodies.
    • Long-lived memory B-cells provide long-term immunity.

Cell-mediated Immunity

  • Aimed at intracellular pathogens:
    • Pathogens: Viruses that infect cells, aberrant cells (cancer).
  • Process:
    • Cytotoxic T cells destroy infected cells.
    • Helper T cells assist in managing the adaptive response.

Antigens

  • Definition: Any substance recognized as foreign by the immune system.
    • Types of Antigens: Bacteria, viruses, fungi, protozoa, drugs, etc.
  • Epitopes: Specific regions of the antigen recognized by antibodies; a single antigen can have multiple epitopes.

Antigen Characteristics

  • Good antigens: High molecular weight, complex structures (e.g. proteins).
  • Haptens: Small molecules that require a larger carrier molecule for immunogenicity (e.g. urushiol).

Antibodies

  • Also known as: Immunoglobulins.
  • Functions:
    • Increase phagocytosis, neutralize toxins, activate complement system.
  • Classes of Antibodies: IgG, IgM, IgA, IgD, IgE.

Antibody Structure

  • Shape: Y-shaped with variable and constant regions.
  • Significance of Variable Region: Binds specific antigens.
  • Function of Constant Region: Determines the class and function of the antibody.

Antibody-Pathogen Interactions

  • Neutralization: Antibodies prevent pathogens from binding to cells.
  • Opsonization: Antibodies tag pathogens for phagocytosis.
  • Agglutination: Antibodies cause clumping of pathogens for easier removal.
  • ADCC: Antibody-dependent cellular cytotoxicity involves NK cells destroying large pathogens.

Major Histocompatibility Complex (MHC)

  • Definition: Molecules that present antigens on cell surfaces.
    • Class I MHC: Present on all nucleated cells; present endogenous antigens.
    • Class II MHC: Present on APCs (macrophages, dendritic cells, B cells); present exogenous antigens.

Antigen Presentation

  • MHC I presents antigens to CD8 T-cells (cytotoxic), while MHC II presents to CD4 T-cells (helper).

T Cells

  • Essential in orchestrating immune responses.
  • Types of T Cells:
    • Helper T cells (CD4): Direct immune responses.
    • Cytotoxic T cells (CD8): Kill infected cells.
    • Regulatory T cells: Prevent autoimmunity.

T Cell Activation

  • Requires interaction with APCs presenting antigen with the corresponding MHC.
  • TCR must recognize the specific MHC-antigen complex.

B Cell Activation

  • Dependent: Interact with T cells for protein antigens.
  • Independent: Can respond directly to non-protein antigens (e.g., polysaccharides).

Immune Response Phases

  • Primary Response: Latency before antibody production; gradual rise in antibody titer, mainly IgM.
  • Secondary Response: Quicker and more robust due to memory cells; mainly IgG produced.

Vaccination and Immunity

  • Active Immunity: Derived from exposure to pathogens or vaccination.
  • Passive Immunity: Transfer of antibodies from mother to infant or through medical treatments.

Herd Immunity

  • Vaccination can lead to herd immunity, reducing effective transmission of diseases within a population.