Antibodies (Immunoglobulins): Structure, Function, and Clinical Role

Antibodies

Antibodies (also immunoglobulins, Ig) are Y-shaped glycoproteins produced by plasma cells (differentiated B lymphocytes) in response to an antigen.

Structure

• Two identical heavy chains, two identical light chains (disulfide-linked).
• Fab region (antigen-binding): Variable domains (VH, VL) confer specificity.
• Fc region (effector): Constant domains; binds Fc receptors, complement.

Five Classes (Isotypes)

Isotype	Primary Role
IgG	Main serum antibody; secondary response; crosses placenta; opsonization, complement activation, ADCC.
IgM	First response; pentamer; potent complement activator; agglutination; B-cell receptor (monomer).
IgA	Mucosal immunity (secretory IgA dimer); prevents pathogen adhesion.
IgE	Type I hypersensitivity (allergy); parasitic defense; binds mast cells/basophils.
IgD	B-cell receptor; role in activation; trace in serum.

Mechanisms of Action

• Neutralization (block pathogen/toxin binding)
• Opsonization (enhance phagocytosis)
• Complement activation (classical pathway)
• Antibody-dependent cellular cytotoxicity (ADCC)
• Agglutination/Precipitation

Antibodies are central to adaptive immunity, diagnostics (serology), and therapeutics (monoclonal antibodies).