Humoral Immune Response

Overview

• Initiated when B cells encounter a target antigen.
• Leads to production of antibodies specific to the antigen.
• Antibodies help mark antigens for destruction.

Activation of B Cells

• B cells activated when antigen binds to receptor on surface.
• Binding causes cross-linking and receptor-mediated endocytosis (clonal selection).
• B cells proliferate and differentiate into effector cells.

Role of T Cells

• Interactions with T cells often required for full activation of B cells.

Plasma Cells

• Most clone cells become plasma cells.
• Plasma cells secrete specific antibodies (up to 2,000/sec) for 4-5 days.

Memory Cells

• Clone cells that do not become plasma cells become memory cells.
• Memory cells enable a rapid response to future exposures to the same antigen.

Primary vs. Secondary Immune Response

• Primary Response:
  • Lag period of 3-6 days after antigen encounter.
  • Peak plasma cell production at 10 days, then declines.
• Secondary Response:
  • Faster and more effective due to memory cells.
  • Peak antibody levels within 2-3 days.
  • Higher antibody levels than primary response.

Active Humoral Immunity

• Naturally Acquired:
  • Formed in response to actual infection.
• Artificially Acquired:
  • Formed in response to vaccination.

Passive Humoral Immunity

• Naturally Acquired:
  • Antibodies delivered via placenta or breast milk.
• Artificially Acquired:
  • Injection of antibodies (e.g., gamma globulin).
  • Used in rapidly fatal diseases.

Antibodies (Immunoglobulins)

Structure

• T or Y-shaped monomers with 4 polypeptide chains.
• Composed of 2 heavy and 2 light chains.
• Variable region: antigen binding site.
• Constant region: determines antibody class.

Classes of Antibodies

• IgM:
  • First released, potent agglutinating agent.
  • Pentamer structure.
• IgA:
  • Found in mucous and secretions.
  • Prevents pathogen entry.
• IgD:
  • Acts as a receptor on B cell surface.
• IgG:
  • Most abundant, secondary and late primary responses.
  • Crosses placental barrier.
• IgE:
  • Involved in allergies and parasitic infections.

Antibody Function

• Do not destroy antigens but mark them for destruction.
• Mechanisms include neutralization, agglutination, precipitation, and complement fixation.

Monoclonal Antibodies

Production and Use

• Commercially prepared, specific to single antigenic determinant.
• Produced by hybridomas (tumor cell and B cell fusion).
• Used in clinical and research applications.