Immunoglobulin Structure and Classes
On this page
Direct answer
Immunoglobulins are glycoproteins built from two identical heavy chains and two identical light chains joined by disulphide bonds, forming a Y-shaped antibody. The arms bind antigen through variable regions, while the stem, or Fc region, determines effector functions. Humans have five classes, IgG, IgA, IgM, IgD and IgE, which differ in heavy chain constant regions and therefore in distribution and function.
What you must remember
- Light chains: kappa and lambda types, with kappa about 60 percent of human antibodies, and each antibody carries only one type.
- Heavy chains: mu, gamma, alpha, delta and epsilon, which define the class of the antibody.
- Fab and Fc: the Fab arms bind antigen through variable domains, while the Fc stem carries effector functions such as complement binding.
- IgG: the main serum antibody, with four subclasses, crossing the placenta through the neonatal Fc receptor and having a half-life of about three weeks.
- IgM: the first antibody in a primary response, a pentamer with a J chain in serum, and the most efficient complement activator.
- IgA: a dimer in secretions joined by a J chain and carrying a secretory component, found in saliva, tears, colostrum and breast milk.
- IgE: binds mast cells and basophils through high-affinity Fc receptors, mediates immediate hypersensitivity and has the lowest serum concentration.
- IgD: a membrane receptor on naive B cells alongside IgM, present at very low levels in serum.
Five classes compared by structure and role
Every antibody is built on the same modular plan, so the classes differ mainly in the heavy chain constant region. Distribution follows function, which makes the classes easy to reason through. The constant region sets the class and its effector behaviour, while the variable domains stay the same within one B cell clone.
| Class | Structure | Main role |
|---|---|---|
| IgG | Monomer, four subclasses | Main serum antibody; crosses placenta |
| IgM | Pentamer with J chain | Early response; strongest complement activator |
| IgA | Dimer in secretions | Mucosal defence; present in colostrum |
| IgD | Monomer on naive B cells | Membrane receptor; very low serum levels |
| IgE | Monomer | Binds mast cells; allergy and helminth defence |
Read the table by asking where each class works, since IgA guards mucosae, IgE sits on mast cells and IgG circulates through blood and tissues. Class switching changes only the constant region of the heavy chain, so the variable region and its antigen specificity are preserved. A primary response therefore starts with IgM and shifts towards IgG through class switching and affinity maturation.
Where the exam tries to catch you
Immunoglobulin questions often hinge on which class is found where, and which one crosses the placenta.
- Trap: equating IgA with the main antibody in blood Correct: IgG dominates serum, while IgA is the main antibody of secretions and works as a dimer with secretory component.
- Trap: assuming IgG marks infection acquired in the womb Correct: IgM is used for intrauterine infection because it does not cross the placenta, so raised IgM in a newborn suggests infection acquired in utero.
- Trap: assuming IgE is abundant in serum because it drives allergy Correct: IgE has the lowest serum concentration, but it is potent because it is bound to mast cells and basophils.
Frequently asked questions
Which immunoglobulin class crosses the placenta?
IgG is the only immunoglobulin class that crosses the placenta efficiently, using the neonatal Fc receptor to transfer maternal antibody to the fetus. This transfer gives the newborn passive immunity during the first months of life.
What is secretory IgA?
Secretory IgA is a dimer of IgA joined by a J chain and carrying a secretory component, which protects it from proteolytic enzymes in mucosal secretions. It is the main antibody in saliva, tears, colostrum and breast milk.
Which antibody class appears first in a primary immune response?
IgM is the first antibody class produced in a primary immune response, appearing within days of antigen exposure. Its pentameric structure gives high avidity, which makes it an efficient early complement activator.
Which immunoglobulin mediates immediate hypersensitivity?
IgE mediates immediate hypersensitivity, because it binds high-affinity Fc receptors on mast cells and basophils. Cross-linking of bound IgE by allergen triggers degranulation and release of histamine and other mediators.
What are the two light chain types and their proportions?
Human light chains are of two types, kappa and lambda, with kappa making up about 60 percent of antibodies and lambda about 40 percent. Each antibody molecule contains two identical light chains, and both are always of one type.