Chapter 21: The Immune System – Innate and Adaptive Body Defenses

Innate Defenses

The immune system protects the body from pathogens through two main types of defenses: innate (nonspecific) and adaptive (specific). Innate defenses provide immediate, general protection, while adaptive defenses target specific threats.

Surface Barriers: The First Line of Defense

• Skin: A highly keratinized epithelial membrane that acts as a physical barrier to most microorganisms, their enzymes, and toxins.

• Mucous Membranes: Line body cavities open to the exterior, trapping pathogens and preventing their entry.

• Protective Chemicals:

  • Acid: Inhibits bacterial growth (e.g., skin, stomach, vagina).

  • Enzymes (Lysozyme): Destroy microorganisms (found in saliva, tears, mucus).

  • Mucin: Forms sticky mucus to trap pathogens.

  • Defensins: Antimicrobial peptides that disrupt microbial membranes.

  • Other Secretions: Specialized regional secretions (e.g., sebum, sweat) with antimicrobial properties.

Innate Internal Defenses: The Second Line of Defense

• Phagocytes:

  • Neutrophils and Macrophages engulf and destroy pathogens.

  • Pathogens are coated with opsonins (complement proteins) to enhance adherence.

  • Destruction occurs via engulfment, acidification of phagolysosome, and digestion with lysosomal enzymes.

  • If unable to ingest, phagocytes may release lethal chemicals.

• Natural Killer (NK) Cells: Lyse and kill cancer cells and virally infected cells; secrete chemicals to enhance inflammation.

• Inflammation:

  • Triggered by injury, infection, or irritants.

  • Benefits: Prevents spread of damage, disposes of debris/pathogens, alerts adaptive immunity, initiates repair.

  • Cardinal Signs: Redness, heat, swelling, pain.

  • Caused by release of inflammatory chemicals (e.g., histamine, kinins, prostaglandins) leading to vasodilation and increased capillary permeability.

  • Neutrophils and macrophages invade the site via leukocytosis, margination, diapedesis, and chemotaxis.

• Antimicrobial Proteins:

  • Interferons: Produced by virally infected cells; induce synthesis of antiviral proteins in neighboring cells.

  • Complement: Group of ~20 plasma proteins; destroy pathogens via classical, lectin, and alternative pathways.

• Fever:

  • Systemic response to infection.

  • Pyrogens (from leukocytes/macrophages) act on hypothalamus to raise body temperature.

  • Higher temperature inhibits bacterial growth and enhances repair.

Adaptive Defenses

Adaptive immunity is specific, systemic, and has memory. It consists of humoral (antibody-mediated) and cellular (cell-mediated) responses.

Key Aspects of Adaptive Immunity

• Specificity: Targets specific antigens.

• Systemic Response: Not limited to infection site.

• Memory: Faster, stronger response upon re-exposure.

Humoral Immunity

• Provided by antibodies produced by B lymphocytes in body fluids.

Cellular Immunity

• Involves direct attack by T lymphocytes; living cells are the protective factor.

Antigens

Antigens are substances that trigger adaptive immune responses. They are recognized as foreign and targeted by the immune system.

• Complete Antigens: Have immunogenicity (stimulate lymphocyte proliferation) and reactivity (react with lymphocytes/antibodies).

• Haptens: Incomplete antigens; not immunogenic unless attached to body proteins.

• Antigenic Determinants: Specific regions of an antigen recognized by antibodies or lymphocytes.

• Self-Antigens: Body's own antigens, not antigenic to self; identified by MHC proteins on cell surfaces.

B and T Lymphocytes & Antigen-Presenting Cells (APCs)

B and T lymphocytes are central to adaptive immunity. Their development ensures immunocompetence (ability to recognize antigens) and self-tolerance (avoid attacking self).

• Origin: Both originate in bone marrow.

• Maturation:

  • B cells: Mature in bone marrow.

  • T cells: Mature in thymus.

• Immunocompetence: Ability to recognize specific antigens.

• Self-Tolerance: Ability to avoid attacking self-antigens.

• Clonal Selection: Antigen binding selects lymphocytes for further development; activated cells divide into effector and memory cells.

• APCs:

  • Dendritic Cells: Phagocytose antigens, present to T cells.

  • Macrophages: Present antigens to T cells, become aggressive phagocytes.

  • B Lymphocytes: Present antigens to helper T cells for activation.

Humoral Immunity: Antibody Production

Humoral immunity targets extracellular antigens through antibody production by B cells.

• Clonal Selection: Antigen binds to B cell receptor, activating the B cell to proliferate and differentiate.

• Plasma Cells: Secrete antibodies specific to the antigen.

• Memory Cells: Provide rapid response upon re-exposure to the same antigen.

Immunological Memory

• Primary Response: First exposure; lag of 3–6 days; antibody levels peak at ~10 days.

• Secondary Response: Subsequent exposure; rapid (hours), higher antibody levels, longer duration.

Active vs. Passive Humoral Immunity

• Active Immunity:

  • Naturally Acquired: Infection.

  • Artificially Acquired: Vaccination.

• Passive Immunity:

  • Naturally Acquired: Maternal antibodies (placenta, breast milk).

  • Artificially Acquired: Injection of preformed antibodies.

Antibody Structure and Classes

• Antibody (Immunoglobulin) Structure: Y-shaped molecule with variable regions for antigen binding.

• Functions: Neutralization, agglutination, precipitation, complement activation.

• Five Classes:

  • IgM: First antibody produced; pentamer; activates complement.

  • IgA: Found in mucous membranes, saliva, tears; protects body surfaces.

  • IgD: Functions as B cell receptor.

  • IgG: Most abundant; crosses placenta; provides passive immunity to fetus.

  • IgE: Involved in allergic reactions and defense against parasites.

Mechanisms of Antibody Action

• Neutralization: Antibodies block harmful effects of toxins or pathogens.

• Agglutination: Antibodies cause pathogens to clump together.

• Precipitation: Soluble antigens are cross-linked and precipitated.

• Complement Activation: Antibody-antigen complexes activate complement, leading to cell lysis.

Key Equations

• Antibody Titer (Concentration):

Example

• Vaccination: Artificially acquired active immunity; stimulates production of memory cells for rapid response upon exposure to the pathogen.

• Maternal Antibodies: IgG crosses placenta, providing passive immunity to the newborn.

Additional info: Adaptive immunity relies on genetic recombination in lymphoid stem cells to generate diverse antigen receptor sites, enabling recognition of a vast array of pathogens.