| Class | General structure | | Location | Function |
|---|
| IgG | Monomer | | Free in blood plasma; about 80 percent of circulating antibodies | Most abundant antibody in primary and secondary immune responses; crosses placenta and provides passive immunization to fetus |
| IgM | Pentamer | | Surface of B cell; free in blood plasma | Antigen receptor on B cell membrane; first class of antibodies released by B cells during primary response |
| IgD | Monomer | | Surface of B cell | Cell surface receptor of mature B cell; important in B cell activation |
| IgA | Dimer | | Saliva, tears, milk, and other body secretions | Protects mucosal surfaces; prevents attachment of pathogens to epithelial cells |
| IgE | Monomer | | Secreted by plasma cells in skin and tissues lining gastrointestinal and respiratory tracts | Binds to mast cells and basophils to sensitize them to subsequent binding of antigen, which triggers release of histamine that contributes to inflammation and some allergic responses |