Topic 11) Immunity (AS)

11.1 Principles of the Immune System

🔸 Types of Immunity

• Innate (non-specific) immunity:

  • Present from birth

  • Immediate response

  • Includes physical and chemical barriers (e.g., skin, stomach acid)

  • Phagocytes (neutrophils and macrophages)

  • Inflammation and fever

• Adaptive (specific) immunity:

  • Slower initial response

  • Specific to particular pathogens

  • Has memory (quicker upon re-exposure)

🔸 Cells Involved

• Phagocytes:

  • Engulf and digest pathogens (phagocytosis)

  • Present antigens on surface (antigen-presenting cells or APCs)

• Lymphocytes:

  • B lymphocytes (B cells):

    • Mature in bone marrow

    • Produce antibodies

    • Involved in humoral immunity

  • T lymphocytes (T cells):

    • Mature in thymus

    • Recognize antigens presented by APCs

    • Involved in cell-mediated immunity

🔸 Types of T cells

• T helper cells (CD4+):

  • Activate B cells and cytotoxic T cells

  • Secrete cytokines

• Cytotoxic T cells (CD8+):

  • Destroy virus-infected or cancerous cells

• Memory T cells:

  • Provide long-term immunity

🔸 Immune Response Sequence

1. Recognition of antigen by B or T cells

2. Clonal selection: Specific lymphocytes activated

3. Clonal expansion: Mitosis to produce identical cells

4. Differentiation into:

   • Plasma cells (B cells)

   • Memory cells

   • Effector T cells

11.2 Antibodies

🔸 Structure of Antibodies

• Immunoglobulins (Ig): Proteins produced by plasma cells

• Basic structure:

  • 2 heavy chains, 2 light chains

  • Variable region (antigen-binding site) — specific to antigen

  • Constant region — determines antibody class and function

  • Hinge region — provides flexibility

🔸 Functions of Antibodies

• Neutralization: Bind to toxins or viruses to prevent harm

• Agglutination: Clump pathogens for easier phagocytosis

• Opsonization: Mark pathogens for destruction

• Complement activation: Lysis of pathogens

🔸 Types of Immunoglobulins (mentioned generally, not detailed in AS)

• IgG, IgA, IgM, IgE, IgD — each with different roles (IgG most common)

11.3 Vaccination

🔸 Definition

• A vaccine contains antigens that stimulate the immune system to produce memory cells without causing disease.

🔸 Types of Vaccines

• Live attenuated (weakened form of the pathogen)

• Inactivated (killed pathogen)

• Toxoid (inactivated toxins)

• Subunit or recombinant (specific proteins or fragments)

🔸 How Vaccines Work

• Introduce antigen → immune response → memory B and T cells

• On exposure to real pathogen:

  • Rapid production of antibodies

  • Faster, stronger immune response (secondary response)

🔸 Herd Immunity

• Occurs when a large proportion of the population is vaccinated

• Reduces spread, protecting unvaccinated individuals

🔸 Vaccination Programs

• Mass immunization schedules (e.g., MMR, polio, COVID-19)

• Importance in global disease control (e.g., eradication of smallpox)

🔸 Challenges in Vaccination

• Antigenic variation (e.g., influenza virus)

• Public misinformation or hesitancy

• Need for boosters in some cases

• Cold chain requirements (for storage/transport)