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[Biochemistry](https://durgical.com/en/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](https://www.rcsb.org/structure/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))](https://durgical.com/en/pharmacology/anti-d-immunoglobulin/)
- [Rabies immunoglobulin](https://durgical.com/en/pharmacology/rabies-immunoglobulin/)
- [Human tetanus immunoglobulin](https://durgical.com/en/pharmacology/human-tetanus-immunoglobulin/)
- [Normal human immunoglobulin (IVIG)](https://durgical.com/en/pharmacology/normal-human-immunoglobulin/)

## Related conditions

- [Multiple myeloma](https://durgical.com/en/conditions/multiple-myeloma/)
- [Amyloidosis](https://durgical.com/en/conditions/amyloidosis/)
- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/)
- [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/)

## Related

- [Plasma proteins and electrophoresis](https://durgical.com/en/biochemistry/topics/plasma-proteins-and-electrophoresis/)
- [Albumin](https://durgical.com/en/biochemistry/enzymes/albumin/)
- [Protein structure and folding](https://durgical.com/en/biochemistry/topics/protein-structure-and-folding/)

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/biochemistry/enzymes/immunoglobulins/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [IGHG1: Immunoglobulin heavy constant gamma 1 (human)](https://www.uniprot.org/uniprotkb/P01857/entry) (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](https://www.rcsb.org/structure/1HZH) (RCSB Protein Data Bank, 2026); [PDB-101: Molecule of the Month: Antibodies](https://pdb101.rcsb.org/motm/21) (RCSB Protein Data Bank (PDB-101), 2026); [Immunoglobulins Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/immunoglobulins-blood-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.

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