CHAPTER 21- PEARSON NOTES

Overview of the Immune System

  • Every second, hostile bacteria, fungi, and viruses challenge our health.

  • Healthy body despite constant exposure due to the immune system's efficiency.

  • The immune system is functional rather than anatomical, involving diverse molecules and trillions of immune cells (especially lymphocytes).

  • Comparison to medieval castle defenses—three lines of defense.

Three Lines of Defense

  1. Surface Barriers

    • First line of defense is intact skin and mucosae.

    • Functions as structural barriers (similar to castle walls and moats).

    • Coated in secretions (e.g., mucus, tears).

  2. Innate Internal Defenses

    • Second line of defense activated when surface barriers are penetrated.

    • Utilizes internal defenses such as:

      • Antimicrobial proteins.

      • Phagocytes (cells that engulf pathogens).

      • Other immune cells.

    • Characteristics:

      • Always present, checks for friend or foe, capable of direct attack and calling reinforcements.

      • Hallmark: inflammation.

  3. Adaptive (Specific) Defense System

    • Third line of defense functions like elite soldiers with specialized weapons.

    • Slower to respond to invaders, must be trained (specific immunity).

    • Adaptive and innate immunity are interconnected; both release and recognize similar molecules, communicate, and assist each other in immune responses.

    • Effective against microorganisms, cancer cells, and transplants, providing both direct attacks and mobilizing chemicals for broader responses.

Part 1: Innate Defenses

  • Innate defenses include mechanical barriers, cells, and chemicals ready from birth.

  • Critical for reducing the workload of the adaptive system by preventing entry and spread of pathogens.

1.1 Surface Barriers

  • Skin and mucous membranes provide first line of defense with keratin as a physical barrier.

  • Characteristics of surface membranes:

    • Acidic secretions: inhibit bacterial growth.

    • Enzymes: Lysozyme destroys bacteria; protein-digesting enzymes kill microorganisms in the stomach.

    • Mucin: forms sticky mucus, trapping microorganisms.

    • Defensins: antimicrobial peptides secreted to help control microbial colonization.

    • Other chemicals: lipid components of sebum and dermcidin in sweat are toxic.

Key Elements of Surface Membranes:

  • Intact epidermis.

  • Mucosal linings (digestive, respiratory, urinary, reproductive).

1.2 Innate Internal Defenses

  • Internal innate defenses consist of cells and chemicals as second line of defense, including:

    • Phagocytes: the primary types are neutrophils (abundant, first responders) and macrophages (voracious eaters).

    • Natural Killer (NK) cells: destroy virus-infected and cancer cells without needing specific antigens. They induce apoptosis in targets.

    • Inflammation: A vital process that includes:

    • Preventing spread of damaging agents.

    • Disposing of debris and pathogens.

    • Alerting adaptive immune response.

    • Setting stage for repair.

Inflammatory Response Characteristics:

  • Four cardinal signs:

    1. Redness

    2. Heat

    3. Swelling

    4. Pain

    • Possible fifth sign: impaired function.

Phagocytosis:

  • Mechanism:

    1. Adhesion: Phagocyte adheres to pathogen.

    2. Engulfment: Membrane surrounds and engulfs pathogen, forming phagosome.

    3. Lysosome fusion: Phagosome fuses with lysosome to form phagolysosome.

    4. Destruction: Digestive enzymes break down external material.

    5. Exocytosis: Expelling indigestible material.

Opsonization:

  • Pathogen “tasty” through opsonins (complements or antibodies) allowing easier phagocytosis.

Phagocyte Mobilization During Inflammation:

  • Leukocytosis: Increase in white blood cells (neutrophils).

  • Margination: Phagocytes adhere to blood vessel walls.

  • Diapedesis: Phagocytes squeeze through capillary walls to reach site.

  • Chemotaxis: Migration towards chemical signals released by damaged cells or pathogens.

  • Neutrophils & Monocytes Role:

    • Neutrophils act quickly as first responders.

    • Monocytes become macrophages, enhancing phagocytosis and tissue repair.

Additional Innate Defenses:

  • Antimicrobial Proteins

    • Interferons: Secreted by infected cells to warn nearby cells.

    • Complement System: Helps lyse pathogens and enhances phagocytosis by opsonization.

Part 2: Adaptive Defenses

  • Adaptive immunity is delayed and requires exploits against specific pathogens or foreign substances (antigens). It provides long-term protection through memory cells.

2.1 Immobilization of Antigen via Antigens

  • Antigen: Molecules that B and T lymphocytes recognize and respond to.

    • Complete Antigen: Has both immunogenicity and reactivity.

    • Hapten: Incomplete antigen that only becomes immunogenic when linked to a body protein.

    • Antigenic Determinants: Portion of an antigen recognized by lymphocytes.

2.2 Lymphocyte Classes

  • Two major classes: B lymphocytes (humoral immunity) and T lymphocytes (cell-mediated immunity).

  • Lymphocytes rely on antigen recognition for activation and proliferation.

Summary of Lymphocytes:

  • B cells produce antibodies and differentiate into memory cells & plasma cells.

  • T cells differentiate into helper T cells, cytotoxic T cells, and regulatory T cells.

2.3 Antigen-Presenting Cells:

  • APCs include dendritic cells, macrophages, and B cells that help activate T cells by presenting processed antigens.

  • Dendritic cells: Key role in antigen capture and presentation, linking innate and adaptive immunity.

Key Points on Vaccination and Immunity:

  • Active Immunity: Developed from exposure to pathogens.

  • Passive Immunity: Temporary immunity from anti-serum or maternal antibodies.

Homeostatic Imbalance:

  • Lack of immune response can lead to diseases like HIV/AIDS, wherein the immune system fails to protect against infection due to lack of T cells.

Autoimmune Disorders:

  • Conditions wherein the immune system attacks own tissues (e.g., rheumatoid arthritis).

Hypersensitivity Reactions:

  • Delayed and immediate hypersensitivities leading to allergic reactions or autoimmune responses.

Summary of Immunity Development:

  • The immune system matures through exposure to antigens and can weaken with age, leading to increased susceptibility to infections and immune disorders.