Understanding Antibody Classes: A Comprehensive Guide

Immunoglobulins (antibodies) are glycoprotein molecules produced by plasma cells. Five classes (isotypes) differ in heavy chain, structure, distribution, and function.

The Five Antibody Classes

Class	Heavy Chain	Structure	Serum Conc.	Key Features & Functions
IgG	γ	Monomer (150 kDa)	80–85% (10–16 mg/mL)	Most abundant; only class crossing placenta (passive immunity); opsonization, complement activation (classical), ADCC, neutralization, feedback inhibition of B cells. Long half-life (~21 days). 4 subclasses (IgG1-4).
IgA	α	Monomer (serum); Dimer + J chain + secretory component (sIgA, mucosa)	10–15% (1–4 mg/mL)	Mucosal immunity: blocks pathogen adhesion, neutralizes toxins/viruses. Secreted in tears, saliva, sweat, colostrum, GI/respiratory/GU tracts. Does not fix complement well. Half-life ~6 days.
IgM	μ	Pentamer + J chain (970 kDa)	5–10% (0.5–2 mg/mL)	First antibody in primary response; pentamer = 10 binding sites → potent agglutination, complement activation (classical). Confined to intravascular (large size). B-cell receptor (monomer). Half-life ~5 days.
IgD	δ	Monomer	<0.25% (<0.04 mg/mL)	Primarily B-cell receptor (co-expressed with IgM); role in B-cell activation, maturation, respiratory immunity. Serum function unclear. Half-life ~3 days.
IgE	ε	Monomer	Trace (<0.0005 mg/mL)	Binds FcεRI on mast cells/basophils. Cross-linking by allergen → degranulation → histamine, leukotrienes → Type I hypersensitivity (allergy, anaphylaxis). Anti-helminth immunity. Half-life ~2 days (serum); weeks (cell-bound).

Clinical Utility of Isotype Testing

• IgG: Immunity status (vaccines, past infection), subclass deficiency workup.
• IgA: Selective IgA deficiency (most common PID), celiac serology (IgA-tTG), mucosal immunity.
• IgM: Acute infection marker (e.g., IgM anti-HAV, -HBV, -CMV, -rubella).
• IgE: Allergy testing (specific IgE), parasitic infections, hyper-IgE syndromes.
• IgD: Rarely measured; elevated in IgD myeloma, periodic fever syndromes.

Structural Basis of Function

• Fab region: Antigen binding (variable domains).
• Fc region: Effector functions (FcγR binding, complement, placental transport, half-life via FcRn).
• Hinge region: Flexibility for antigen access.

Understanding isotype biology guides vaccine design, immunotherapy, and immunodeficiency diagnosis.