Immunology · Adaptive immunity
Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD) !
- Molecule
Antibodies are Y-shaped proteins; their tips grab one target and their tail decides what happens next, which differs among the five classes.
Updated 2026-11-01
What it is
An antibody (immunoglobulin) has two identical heavy chains and two identical light chains joined by disulfide bonds. The Fab arms carry the variable regions that bind the epitope; the Fc stem, defined by the heavy-chain constant region, determines the isotype and its effector functions, such as complement activation, binding to phagocytes, crossing the placenta or reaching mucosal secretions. Humans have five isotypes: IgG, IgA, IgM, IgE and IgD.
Structure
- Two heavy chains (gamma, alpha, mu, epsilon or delta) and two light chains (kappa or lambda).
- Fab: antigen-binding fragment, with variable domains of heavy and light chains and their hypervariable loops (CDRs).
- Fc: crystallizable fragment, binds Fc receptors and C1q.
- Hinge region (IgG, IgA, IgD) gives flexibility to the arms.
- J chain links IgM pentamers and IgA dimers; secretory component accompanies IgA across epithelium.
The five isotypes
| Isotype | Form | Where it predominates | Key functions | Crosses placenta / activates complement |
|---|---|---|---|---|
| IgG | Monomer | Blood and tissues; most abundant in serum | Opsonization, neutralization, NK cell killing (ADCC); main antibody of the secondary response | Yes / Yes (classical) |
| IgA | Monomer in serum, dimer in secretions | Mucosa, saliva, tears, breast milk | Neutralizes microbes at mucosal surfaces without inflammation | No / Poorly |
| IgM | Pentamer in serum; monomer on naive B cells | Blood (too large to leave easily) | First antibody of the primary response; best complement activator; agglutination | No / Yes, very strongly |
| IgE | Monomer | Bound to mast cells and basophils (lowest serum level) | Immediate allergy, anaphylaxis, defense against helminths | No / No |
| IgD | Monomer | Surface of naive B cells | Antigen receptor together with IgM; secreted role unclear | No / No |
Clinical notes
- Selective IgA deficiency is the most common primary immunodeficiency; most patients are asymptomatic, some have sinopulmonary or GI infections, and anti-IgA antibodies can cause anaphylaxis to blood products.
- Anti-D IgG crosses the placenta in an Rh-negative mother sensitized to Rh-positive red cells and can cause hemolytic disease of the fetus and newborn.
- In serology, specific IgM suggests recent infection and IgG suggests past infection or immunity.
- Monoclonal antibodies used as drugs are mostly IgG.
In the lab
- Serum protein electrophoresis shows immunoglobulins in the gamma region; a narrow spike suggests a monoclonal protein.
- Quantitative IgG, IgA and IgM are measured when antibody deficiency is suspected.
Test yourself
Think of the answer, then tap the question to see it.
Which antibody class crosses the placenta?
IgG, through the neonatal Fc receptor (FcRn) on placental cells.
Which isotype is the most efficient activator of the classical complement pathway, and why?
IgM: as a pentamer it offers several Fc regions close together, so one molecule can bind C1q.
What part of the antibody determines its isotype and effector function?
The constant region of the heavy chain, which forms the Fc fragment.
What is the most common primary immunodeficiency, and what risk does it carry in transfusion?
Selective IgA deficiency; some patients have anti-IgA antibodies and can have anaphylaxis with blood products that contain IgA.
Which immunoglobulins use the J chain?
Pentameric IgM and dimeric IgA.
Related
- B-cell activation, class switching and affinity maturation
- Complement: three pathways, MAC, regulators
- Primary vs secondary response and memory
- Neonatal immunity: maternal IgG and breast-milk IgA
- B cell and plasma cell
- Monoclonal antibodies and biologics: naming and targets
- Serology: IgM vs IgG, seroconversion, avidity
- B-cell (antibody) deficiencies: XLA, CVID, IgA deficiency, hyper-IgM
- Type I hypersensitivity (immediate, IgE)
- ABO and Rh blood groups, hemolytic disease of the newborn
Related conditions
Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.
Sources: 18.1 Overview of Specific Adaptive Immunity (OpenStax (Microbiology), 2016); 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies (OpenStax (Anatomy and Physiology 2e), 2022).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.