Antibody-Mediated Immunity (Humoral Immunity)

Overview

• Antibody-Mediated Immunity: Also known as humoral immunity.
  • Governed by B cells.
  • Involves production of antibodies by B cells when they encounter antigens.

Primary Response

• Initial Encounter with Antigen:

  • Can occur naturally (e.g., flu infection) or artificially (e.g., flu vaccine).
  • B cells recognize antigen and produce antibodies.

• Types of B cells:

  • Plasma Cells: Rapidly produce antibodies.
  • Memory B Cells: Provide long-term immunity by remembering the antigen.

• Example: Getting a flu vaccine in the fall triggers the primary response.

Secondary Response (Anamnestic Response)

• Role of Memory B Cells:

  • Recognize previously encountered antigens more rapidly.
  • Bigger and faster production of antibodies compared to the primary response.

• Example: Actual flu exposure after vaccination leads to a secondary response.

• Benefit: Faster response may prevent illness or reduce severity.

Functions of B Cells

• Antibody Production:

  • Plasma cells produce about 2000 antibodies per second.
  • Antibodies bind to specific epitopes on antigens.

• Antigen Presentation:

  • B cells display antigens to T cells.
  • T cells inspect and confirm antigens are foreign before B cells clone.

Types of Antibodies (Immunoglobulins)

• Immunoglobulin Types: IGG, IGA, IGM, IGD, IgE.
• Structure: Y-shaped proteins with variable regions for antigen binding.

Types

• IGG: Main immunoglobulin in blood; crosses placenta.
• IGA: Found in secretions (e.g., mucus membranes).
• IgE: Involved in allergic reactions; binds to basophils/mast cells.
• IGM: Largest, first antibody made by the fetus; primary response.
• IGD: Rarely secreted, less common.

Process of Antibody Action

• Primary vs. Secondary Response:

  • Primary: Initial exposure leads to antibody production and memory cells.
  • Secondary: Faster, larger antibody production upon re-exposure.

• Antibody Encounters with Pathogens:

  • IGA: Found in mucous membranes, first line of defense.
  • IgE: Releases histamine to recruit immune cells.
  • Macrophages: Present gobbled antigens to B and T cells.

Mechanisms of Antibody Action

• Agglutination: Clumping of pathogens for easier phagocytosis.
• Opsonization: Coating pathogens to enhance phagocytosis.
• Complement Activation: IgM and IGG bind complement proteins, enhancing inflammation.
• Neutralization: Antibodies prevent pathogen binding to cells.
• Cell Lysis: IGM can punch holes in bacterial membranes.

Next Topic

• Cell-Mediated Immunity: Governed by T cells, covering next.