Antibody Functions, Diversity, and Lymphocyte Roles

Antibody Functions

• Precipitation: Binds to soluble molecules so they become insoluble.
• Neutralization: Binds to and prevents the activity of viruses or toxins.

Fc Region Binding

• Phagocytes: Facilitates opsonization and stimulates cytokine release.
• Complement proteins: Stimulates complement fixation, which triggers MAC formation.

Diversity of Antibodies

• Each person has approximately 10²⁰ antibody molecules with millions of different specificities.
• It is likely that an antibody exists for any antigen a person might encounter.
• Mechanisms of diversity:
  • Genetic recombination: Combinations of hundreds of genes coding for heavy and light chains lead to millions of unique antibodies.
  • Somatic hypermutation: Diversity increases via a high rate of single base pair mutations occurring as B cells proliferate in secondary lymphoid tissues in response to foreign antigens.

Immunoglobulin Classes

• IgG: The most prevalent antibodies in serum; they provide naturally acquired passive immunity, neutralize bacterial toxins, participate in complement fixation, and enhance phagocytosis.
• IgM: Consist of five monomers held by a joining chain; they are involved in agglutination and complement fixation.
• IgA: Serum IgA antibodies are monomers, while secretory IgA antibodies are dimers that protect mucosal surfaces from pathogen invasion.
• IgD: Found on B cells; they may delete B cells that produce antibodies against self.
• IgE: Bind to mast cells and basophils and are involved in allergic reactions.

Lymphocytes

• Lymphocytes are derived from stem cells in bone marrow, which replace themselves by cell division to prevent depletion.
• Produced lymphocytes seed the thymus, spleen, and lymph nodes with self-replacing colonies.

T Lymphocytes (T Cells)

• Develop from lymphocytes that seed the thymus.
• Do not secrete antibodies.
• Attack infected host cells, cancer cells, and foreign cells.
• Provide cell-mediated immunity.

B Lymphocytes (B Cells)