In-Depth Notes on Adaptive Immunity

Adaptive Immunity Overview

  • Definition: Adaptive immunity refers to defenses that specifically target pathogens.
  • Acquisition: Develops through infection or vaccination.
  • Responses:
    • Primary Response: First encounter with a pathogen, involving a slower, more gradual response.
    • Secondary Response: Recurring interaction with a pathogen is quicker and more effective due to immune memory.

Dual Nature of the Adaptive Immune System

  • Two Types:
    1. Humoral Immunity:
    • Involves antibodies targeting foreign molecules (antigens).
    • B Cells: Mature in red bone marrow, recognize antigens, and produce antibodies.
    1. Cellular Immunity:
    • Involves T cells recognizing processed antigens presented by other cells.
    • T Cells:
      • Mature in the thymus.
      • Secrete cytokines rather than antibodies when activated.

Functions of Humoral vs. Cellular Immunity

  • Cellular Immunity: Directly attacks antigens that have invaded cells (e.g., viruses).
  • Humoral Immunity: Addresses threats external to cells (e.g., toxins, extracellular bacteria).

Cytokines: Immune Chemical Messengers

  • Definition: Cytokines are signaling proteins involved in immune responses.
  • Key Types:
    • Interleukins (IL): Mediate communication between leukocytes.
    • Chemokines: Involved in cell movement and migration.
    • Interferons (IFNs): Antiviral agents.
    • Tumor Necrosis Factor Alpha (TNF-α): Involved in systemic inflammation.
  • Cytokine Storm: Excessive production can lead to severe inflammation.

Antigens and Antibodies

  • Antigens: Substances that trigger an immune response by producing antibodies.
    • Found on microbes or foreign cells.
    • Antibodies interact with specific parts (epitopes) of antigens.

Structure of Antibodies

  • Immunoglobulins (Ig): Antibodies composed of four protein chains (two light and two heavy) forming a Y shape.
  • Regions:
    • Variable (v) Regions: Bind to specific epitopes.
    • Constant (Fc) Region: Identifies the class of the antibody.
  • Classes of Antibodies:
    1. IgG: Most abundant, crosses placenta, protects fetus.
    2. IgM: First antibody in response to infection.
    3. IgA: Secreted in mucous membranes; protects surfaces.
    4. IgD: Not well understood, found on B cell surfaces.
    5. IgE: Involved in allergic reactions and defense against parasites.

Activation of B Cells and Antibody Production

  • Activation Process:
    1. B cells recognize and bind to antigens through surface immunoglobins.
    2. Internalization of antigen leads to processing and presentation on MHC class molecules.
    3. T helper cells stimulate B cell activation through cytokines.
    4. Activated B cells undergo clonal expansion, resulting in plasma cells (producing antibodies) and memory cells.

Mechanisms of Antigen-Antibody Interaction

  • Antigen-Antibody Complex: Formed when antibodies bind antigens.
  • Functional Outcomes:
    • Agglutination: Clumping of pathogens for easier targeting by immune cells.
    • Opsonization: Enhancing phagocytosis by coating pathogens with antibodies.
    • Neutralization: Blocking of pathogen attachment to host cells.
    • Complement Activation: Leading to cell lysis and inflammation.

Cellular Immune Response Processes

  • T Cell Activation: Involves recognition of antigens presented by antigen-presenting cells (APCs).
  • Classes of T Cells:
    1. T Helper Cells: Activate other immune cells including B cells and cytotoxic T cells.
    2. Cytotoxic T Lymphocytes (CTL): Induce apoptosis in infected cells.
    3. T Regulatory Cells: Help maintain immune tolerance and prevent autoimmunity.

Immunological Memory and Types of Adaptive Immunity

  • Memory Response: Faster and stronger upon re-exposure to the same antigen due to memory cells.
  • Types of Immunity:
    • Naturally Acquired Active Immunity: Immunity developed after infection.
    • Naturally Acquired Passive Immunity: Antibodies received from mother via placenta or breast milk.
    • Artificially Acquired Active Immunity: Immunity gained through vaccination.
    • Artificially Acquired Passive Immunity: Immunity provided by injecting preformed antibodies.