Antibodies

Definition

Antibodies, also known as immunoglobulins (Ig), are glycoproteins produced by plasma cells in response to antigens. They play a central role in humoral immunity by neutralizing pathogens, facilitating phagocytosis, and activating the complement system. There are five major classes of antibodies, each with distinct structures, locations, and clinical relevance.
1. IgGMost abundant antibody in serum

  • Primary functions:
    • Neutralization of toxins and viruses
    • Opsonization
    • Complement activation
  • Key features:
    • Crosses the placenta → passive immunity to fetus
    • Long half-life (~21 days)
  • Clinical relevance:
    • Main antibody measured for immune status
    • Therapeutic monoclonal antibodies are mostly IgG-based
    • IVIG products are predominantly IgG

2. IgAPrimary antibody in mucosal immunity

  • Locations:
    • Saliva
    • Tears
    • Breast milk
    • Respiratory and gastrointestinal secretions
  • Key features:
    • Exists as a dimer in secretions (secretory IgA)
  • Clinical relevance:
    • Protects mucosal surfaces from pathogen adherence
    • IgA deficiency → recurrent mucosal infections
    • Important consideration in IVIG-related anaphylaxis

3. IgMFirst antibody produced in acute infection

  • Primary functions:
    • Early immune response
    • Potent complement activation
  • Key features:
    • Pentameric structure
    • Does not cross the placenta
  • Clinical relevance:
    • Marker of recent or acute infection
    • Elevated in early immune responses and Waldenström macroglobulinemia

4. IgEKey antibody in allergy and parasitic infections

  • Primary functions:
    • Mediates type I hypersensitivity reactions
    • Binds to mast cells and basophils
  • Key features:
    • Triggers histamine release upon antigen exposure
  • Clinical relevance:
    • Elevated in allergic disorders (asthma, eczema)
    • Target of biologics (e.g., omalizumab)

5. IgDLeast abundant and least understood antibody

  • Primary functions:
    • Regulation of B-cell activation
  • Key features:
    • Primarily membrane-bound on naïve B cells
  • Clinical relevance:

Pharmacist’s Clinical Pearls

  • IgG dominates therapeutic antibody design due to stability and long half-life
  • IgM positivity without IgG suggests recent infection
  • IgA deficiency is the most common primary immunodeficiency
  • Hypersensitivity reactions may involve IgE-mediated mechanisms
  • Vaccine response assessment typically measures IgG titers