Antibodies (Immunoglobulins): Structure, Functions, and Clinical Roles

Antibodies are Y-shaped glycoproteins produced by plasma cells (differentiated B lymphocytes) that specifically bind antigens. Key attributes:

1. Synonym: Immunoglobulin (Ig).
2. Antigen Specificity: Each clone recognizes a unique epitope.
3. Neutralization: Blocks pathogen entry/toxin action.
4. Opsonization: Tags pathogens for phagocytosis (Fc receptor binding).
5. Complement Activation: Classical pathway (IgM, IgG) → lysis, inflammation.
6. ADCC: NK cells kill antibody-coated targets via FcγRIII.
7. Isotypes: IgG (serum, placental transfer), IgM (early response, pentamer), IgA (mucosal, breast milk), IgE (allergy, parasites), IgD (B-cell receptor).
8. Maternal Transfer: IgG crosses placenta (fetal protection); IgA in colostrum/breast milk.
9. Vaccine-Induced: Active immunization generates memory B cells and high-affinity IgG.
10. Pathology: Autoantibodies (autoimmune disease), paraproteins (myeloma), immune complexes (vasculitis).

Therapeutic antibodies (monoclonals) now treat cancer, autoimmune diseases, infections, and more.