4.2 The Adaptive Immune System

Adaptive Immune System

  • Activates when innate defenses fail; acts as the body’s 2nd2^{\text{nd}}/3rd3^{\text{rd}} line of defense.
  • Employs highly trained, relatively scarce B & T lymphocytes ("Special Forces").
  • Key features:
    • Specificity – targets particular antigens.
    • Systemic – operates body-wide, not just at infection site.
    • Memory – quicker, stronger reaction on re-exposure.

Branches of Adaptive Immunity

  • Humoral Immunity
    • Fights extracellular pathogens in body fluids (blood, interstitial fluid).
    • Mediated by B cells → secrete antibodies that tag/neutralize threats.
  • Cell-Mediated Immunity
    • Destroys infected or abnormal host cells (e.g., virus-infected, cancerous).
    • Driven mainly by T cells (plus some NK cell support).
  • Branches cooperate: the same pathogen may require both responses (e.g., viruses outside & inside cells).

Primary vs. Secondary Response

  • Primary Response (first exposure)
    • Time-consuming (several days) – immune system learns antigen, crafts pathogen-specific antibodies & T-cell receptors.
    • Generates memory B & T cells.
  • Secondary Response (subsequent exposure)
    • Memory cells recognize antigen immediately.
    • Rapid, amplified reaction often halts pathogen before illness manifests.
    • Basis of vaccination: vaccine mimics primary exposure without disease, so real pathogen meets a secondary response.

Key Lymphocytes

  • B Cells
    • Produce antibodies (immunoglobulins) that bind, label, neutralize antigens.
  • T Cells
    • Helper T (Th): coordinate immune activities, activate B cells & other leukocytes via cytokines.
    • Cytotoxic T (Tc): directly kill infected/aberrant cells.
    • Regulatory mechanisms prevent overreaction.

Immune System Malfunctions

  • Immunodeficiency (weak/absent response)
    • Inherited or acquired (e.g., HIV → AIDS) – chiefly affects lymphocytes.
  • Autoimmune Disorders (hyperactive, self-targeting)
    • Examples: lupus, rheumatoid arthritis, multiple sclerosis, type 11 diabetes.
  • Allergies (hypersensitivity to non-threat environmental antigens)
    • Symptoms stem from misguided immune attack on allergens (pollen, dander, foods).

Summary Points

  • Adaptive immunity learns, tailors, and memorizes defenses against specific threats.
  • Humoral = extracellular; Cell-mediated = intracellular.
  • Memory enables fast, potent secondary responses; underpin vaccine effectiveness.
  • Dysfunctions present as immunodeficiencies, autoimmunity, or allergies.