Biochemistry · Protein
Immunoglobulins (antibodies) !
Immunoglobulins are antibodies: Y-shaped proteins made by plasma cells to recognise and tag anything foreign.
Updated 2026-11-01
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Key facts
- Two identical heavy chains and two identical light chains (κ or λ), joined by disulfide bridges; Y shape with a flexible hinge.
- Fab (arms): variable regions that recognise antigen (CDRs). Fc (stem): activates complement and binds cell Fc receptors.
- IgG: the most abundant (≈ 75 %), crosses the placenta, secondary response. IgM: pentamer, first to appear, best complement activator.
- IgA: dimer with J chain and secretory component in mucosa, saliva and milk. IgE: allergy and parasites (mast cells). IgD: receptor of naïve B cells.
- On electrophoresis they run in the γ region; a single clone produces an M spike (monoclonal gammopathy).
- Also called
- Antibodies, IgG, IgA, IgM, IgE, IgD, Gamma globulins
- Genes
- IGH, IGK, IGL
- Structure (PDB)
- 1HZH
Structure
- Structure 1HZH is a complete human IgG1 (the anti-HIV antibody b12): two heavy chains (H, K) and two light chains (L, M); the Y shape with both Fabs and the Fc is visible.
- Each chain is built from immunoglobulin-like domains (≈ 110 amino acids, a β-sheet sandwich); the Fc carries a sugar at Asn297 of each heavy chain.
Regulation
- V(D)J recombination and somatic hypermutation create diversity; class switching (from IgM to IgG, IgA or IgE) depends on T-cell signals (CD40L, cytokines).
- IgG is recycled by the FcRn receptor (half-life ≈ 3 weeks).
Inhibitors and toxins
- Immunosuppressants and anti-CD20 antibodies (fewer B cells, less production)
- Renal or gut losses (nephrotic syndrome, protein-losing enteropathy)
Clinical notes
- Multiple myeloma and MGUS: a monoclonal immunoglobulin or light chain (M spike, Bence Jones protein in urine).
- Immunodeficiencies: X-linked agammaglobulinaemia (no B cells), selective IgA deficiency (the commonest), common variable immunodeficiency.
- Monoclonal light chains can form amyloid (AL amyloidosis) or damage the renal tubule (myeloma kidney).
- Maternal IgG protects the infant for the first months; IgM in a newborn suggests congenital infection, because it does not cross the placenta.
Drugs that act here
- Anti-D immunoglobulin (Rho(D))
- Rabies immunoglobulin
- Human tetanus immunoglobulin
- Normal human immunoglobulin (IVIG)
Related conditions
- Multiple myeloma
- Amyloidosis
- HIV infection and AIDS
- Adenosine deaminase deficiency (ADA-SCID)
- Anaphylaxis
Related
Test yourself
Think of the answer, then tap the question to see it.
Which immunoglobulin crosses the placenta?
IgG, carried by the placental FcRn receptor; it protects the infant during the first months.
Which part of an antibody recognises antigen and which activates complement?
The Fabs (variable regions, CDRs) recognise antigen; the Fc activates complement and binds Fc receptors.
Which immunoglobulin appears first in an infection, and why does it activate complement so well?
IgM: it is a pentamer with many Fc regions together, so a single bound molecule can activate C1q.
What is Bence Jones protein?
Monoclonal free light chains (κ or λ) that pass into the urine; typical of multiple myeloma.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: IGHG1: Immunoglobulin heavy constant gamma 1 (human) (UniProt, 2026); 1HZH: CRYSTAL STRUCTURE OF THE INTACT HUMAN IGG B12 WITH BROAD AND POTENT ACTIVITY AGAINST PRIMARY HIV-1 ISOLATES: A TEMPLATE FOR HIV VACCINE DESIGN (RCSB Protein Data Bank, 2026); PDB-101: Molecule of the Month: Antibodies (RCSB Protein Data Bank (PDB-101), 2026); Immunoglobulins Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.