Immunoglobulin Classes

Mnemonic

“GAMED”, in order of serum concentration:

• IgG - most abundant, the only one crossing the placenta, and the antibody of the secondary response
• IgA - mucosal secretions, breast milk and saliva, secreted as a dimer with a secretory component
• IgM - a pentamer, the first antibody made in a primary response, the best complement fixer, and the antibody of the ABO system
• IgE - bound to mast cells, mediating type I hypersensitivity and the response to parasites
• IgD - a B cell surface receptor, function largely unclear

“M for first and Massive, G for the Greatest amount and Gets through the placenta, A for Aqueous secretions, E for Everything allergic.”

Class switching from IgM to the others happens in the germinal centre, requires T cell help through CD40 ligand, and its failure produces hyper-IgM syndrome.

Expansion

• IgG - the most abundant (about 75 per cent). The main secondary response antibody, opsonises, fixes complement, and is the only class to cross the placenta, giving passive immunity for the first 6 months
• IgA - the main mucosal antibody, secreted as a dimer with a secretory component; present in breast milk, especially colostrum
• IgM - a pentamer, the first antibody in a primary response, and the most efficient at fixing complement. Too large to cross the placenta, so its presence in a neonate indicates congenital infection
• IgE - binds mast cells and basophils, mediating type I hypersensitivity and defence against parasites
• IgD - a B cell surface receptor; function otherwise unclear

Two clinical uses of this list: IgM positive serology indicates recent or acute infection while IgG indicates past exposure or immunity; and selective IgA deficiency, the commonest primary immunodeficiency, causes recurrent mucosal infection and a risk of anaphylaxis to blood products containing IgA.