Types of Antibodies (Immunoglobulins)
Compact reference for IgG, IgA, IgM, IgE, and IgD — structure, valency, distribution, half-life, functions, complement, Fc receptors, and clinical notes.
Reference Diagram
Comparison Table
| Isotype | Heavy Chain | Structure / Valency | Distribution | Half-life | Key Functions | Complement | Fc Receptors | Clinical Notes |
|---|---|---|---|---|---|---|---|---|
| IgG | γ (gamma) | Monomer / valency 2 | Blood & interstitial fluid; crosses placenta | ~21 days | Neutralization; opsonization; ADCC (via NK); dominant in secondary responses | Strong (IgG1, IgG3 via classical pathway) | FcγRI/II/III on phagocytes & NK; FcRn (placental transfer) | Maternal–fetal immunity; IVIG therapy; most therapeutic mAbs are IgG |
| IgA | α (alpha) | Monomer (serum) / Dimer (secretions) via J chain | Mucosal secretions (saliva, tears, GI/respiratory, milk); serum | ~6 days | Mucosal immunity; immune exclusion of pathogens/toxins at entry sites | Minimal (does not strongly fix complement) | FcαRI (CD89); pIgR transports dimeric IgA across epithelium | Selective IgA deficiency; secretory IgA supports neonatal gut immunity |
| IgM | μ (mu) | Pentamer (J chain); high avidity | Blood; early humoral response; naive B-cell BCR (monomeric) | ~5 days | First produced in primary response; potent agglutination | Very strong (best classical pathway activator) | FcμR | Marker of recent infection; Waldenström macroglobulinemia; cold agglutinins |
| IgE | ε (epsilon) | Monomer / valency 2 | Bound to FcεRI on mast cells & basophils; mucosal tissues | ~2 days serum (longer cell-bound) | Type I hypersensitivity; parasite defense; triggers degranulation (histamine, LTC4/D4/E4) | Does not fix classical complement | FcεRI (high-affinity), FcεRII (CD23) | Allergy testing; anti-IgE biologics (e.g., omalizumab) reduce exacerbations |
| IgD | δ (delta) | Monomer / valency 2 | Co-expressed with IgM as BCR on naive B cells; trace serum | ~3 days | Modulates B-cell activation and tolerance; limited soluble effector roles | Negligible | Putative FcδR (less defined) | Primarily a phenotypic marker in immunophenotyping |
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At-a-Glance Notes
- IgG: Systemic workhorse; best for opsonization/ADCC; crosses placenta (FcRn).
- IgA: Mucosal shield; dimeric in secretions (secretory component protects from proteolysis).
- IgM: First responder; strongest complement activation; great at agglutination.
- IgE: Allergies & parasites; mast cell/basophil degranulation via FcεRI.
- IgD: B-cell receptor partner; marker of naive B cells.