Innate Immunity

INNATE IMMUNITY OVERVIEW

1. Definition

  • Susceptibility: Lack of resistance to a disease.

  • Immunity: Ability to ward off disease.

  • Innate Immunity: Defenses against any pathogen that does not require prior exposure.

  • Adaptive Immunity: Immunity or resistance to a specific pathogen.

2. Lines of Defense

2.1 First Line of Defense

  • Physical Barriers:

    • Intact skin

    • Mucous membranes and their secretions.

    • Normal microbiota (compete with pathogens).

  • Physical Factors:

    • Mucus: Traps microbes.

    • Ciliary Escalator: Moves trapped microbes away from lungs.

    • Lacrimal Apparatus: Washes eyes.

    • Saliva: Washes microbes off mouth.

    • Urine & Vaginal Secretions: Flows out, helping to eliminate microbes.

  • Chemical Factors:

    • Seba: Contains fungistatic fatty acids.

    • Skin pH: Low pH (3-5) inhibits bacterial growth.

    • Lysozyme: Found in perspiration, tears, saliva, and urine; breaks down bacterial cell walls.

    • Gastric Juice pH: Very low (1.2-3.0), kills microbes.

2.2 Second Line of Defense

  • Phagocytic Cells:

    • Neutrophils, macrophages, eosinophils: Engulf and digest pathogens.

  • Inflammation:

    • Response to tissue damage, characterized by red, swollen, painful areas.

  • Fever: Abnormally high body temperature enhances immune response.

  • Antimicrobial Substances:

    • Complement proteins and interferons enhance the immune response.

3. Cell Types in Immunity

3.1 Leukocytes (White Blood Cells)

  • Types of Leukocytes: 5000-10,000 per ul or mm³

    • Granulocytes:

      • Neutrophils (60-70%): Phagocytosis.

      • Eosinophils (2-4%): Combat parasitic infections and some phagocytosis.

      • Basophils (0.5-1%): Involved in inflammatory responses (release histamine).

    • Agranulocytes:

      • Monocytes (3-8%): Differentiate into macrophages and dendritic cells to perform phagocytosis.

      • Lymphocytes (20-25%): Natural killer (NK) cells destroy infected or cancerous cells; T cells and B cells participate in adaptive immunity.

4. Phagocytosis Process

  • Steps:

    1. Chemotaxis and adherence of phagocyte to microbe.

    2. Ingestion of microbe via pseudopods forming a phagosome.

    3. Fusion of phagosome with lysosome, forming a phagolysosome.

    4. Digestion of ingested microbes by lysosomal enzymes.

    5. Formation of residual body with indigestible material.

    6. Discharge of waste products.

5. Inflammation Process

  • Components:

    • Activation of acute-phase proteins (complement, cytokines).

    • Blood vessel changes (vasodilation, increased permeability from histamine and kinin).

    • Symptoms: Redness, swelling, pain, heat.

    • Phagocyte recruitment (margination and diapedesis).

6. Fever

  • Mechanism:

    • Caused by release of interleukin-1 (IL-1) from phagocytes responding to pathogens.

    • Hypothalamus adjusts body temperature set-point.

    • Benefits: Enhances immune activity and inhibits pathogen growth.

    • Risks: High fever can lead to serious complications (e.g., tachycardia, dehydration).

7. Complement System

  • Function:

    • Series of serum proteins activated in a cascade.

    • Three pathways: Classical, Alternative, and Lectin.

    • Outcomes: Opsonization (enhanced phagocytosis), inflammation, cell lysis (MAC).

8. Interferons

  • Role:

    • IFN-alpha and IFN-beta: Induce antiviral protein production in neighboring cells.

    • IFN-gamma: Enhances phagocytosis by neutrophils and macrophages.