lecture recording on 11 March 2025 at 14.10.42 PM

Active vs. Passive Immunity

• Definitions: Distinguishing between active and passive immunity helps understand immune responses.

  • Active Immunity: Develops when memory cells are formed after exposure to a pathogen, resulting in long-lasting protection.

  • Passive Immunity: Involves passive transfer of antibodies without the generation of memory cells.

Types of Active Immunity

Naturally Acquired Active Immunity

• Occurs when a person contracts a disease and their immune system produces memory cells as a response.

• The individual may feel sick, but the body learns to recognize the pathogen for future defenses.

Artificially Acquired Active Immunity

• Achieved through vaccination, where the body is exposed to a harmless form of the pathogen.

• Vaccines stimulate the immune system to produce memory cells without causing illness. Examples include MMR, COVID, flu, and pneumonia vaccines.

Passive Immunity

Naturally Acquired Passive Immunity

• Occurs when antibodies are transferred from mother to child during breastfeeding.

• The child receives antibodies but does not produce memory cells, providing temporary protection against diseases mother is immune to.

Artificially Acquired Passive Immunity

• Less common; involves administering antibodies directly to an individual who is severely ill to provide immediate protection.

• As no memory cells are produced, this immunity is temporary.

Structure and Function of Antibodies

• Antibody Structure: Commonly depicted as a Y-shaped structure composed of heavy and light chains.

  • Chains: Heavy chains (blue) and light chains (black).

  • Variable Region: Areas on the chains that change, enabling binding to specific antigens.

  • Constant Region: Determines the type of antibody (e.g., IgG, IgA, IgM).

Antibody Types and Their Functions

1. IgG

   • Most abundant antibody in blood, key in secondary immune response.

   • Protects against bacteria and viruses in the bloodstream.

2. IgM

   • First antibody produced during primary immune response, exists as a pentamer.

   • Can bind up to 10 antigens and activates the complement system effectively.

3. IgA

   • Present in secretions like saliva and breast milk, helps prevent pathogen adherence.

   • Exists as a dimer (two subunits).

4. IgE

   • Involved in allergic reactions, binds to mast cells, triggers release of histamine.

   • Found in very low concentrations in plasma.

Mechanisms of Action of Antibodies

• Antigen-Antibody Complex: Formed when an antibody binds to its specific antigen, leading to several immune responses:

  • Complement Activation: Triggers cell lysis, enhances inflammation, and promotes phagocytosis.

  • Neutralization: Antibodies surround and mask dangerous parts of pathogens, preventing their harmful effects.

  • Agglutination: Antibodies cause cells to clump together; can be dangerous when clumping occurs in red blood cells.

  • Precipitation: Small, soluble antigens clumped by antibodies enhance phagocytosis.

B Cell Activation and Clonal Selection

• B cells bind free antigens and require cytokine (IL-2) for full activation.

• Antigen binding leads to proliferation into memory and plasma cells, essential for effective immune response.

  • Plasma Cells: Produce antibodies, key effector cells in the immune response.

T Cell Activation and Types

• T cells require presented antigens via major histocompatibility complex (MHC).

  • Helper T Cells (CD4+): Respond to exogenous antigens, aiding B cells and other immune cells.

  • Cytotoxic T Cells (CD8+): Target cells presenting endogenous antigens, such as virus-infected or cancerous cells.

Summary of Antigen Presentation

• APCs (Antigen Presenting Cells) like dendritic cells process pathogens and present antigens via MHC to activate T cells.

  • This dual binding event allows T cells to recognize and initiate an immune response against foreign elements.