There are several different isotypes of heavy chain constant regions, broken into classes and subclasses. Classes are differentiated by large structural differences correlated to large functional differences. Subclasses have small but significant differences, also corresponding to separate functions. Most functions of antibodies are mediated (determined) by the heavy chain constant region. However, all antibody functions are triggered only by binding of an antigen to the variable region. The two light chain isotypes (κ and λ) associate with all the different heavy chain isotypes. Each isotype is encoded by a separate gene, and all genes are present in normal individuals.
|Isotype||Heavy Chain||Structure & Function||Subclasses (Human)|
|IgM||μ||The IgM heavy chain has four constant regions and no hinge, represented as (H2L2)6. However, a J chain is frequently produced as well to create a (H2L2)5J antibody. IgM has great valency, allowing it to avidly bind antigens and be the first antibody to get produced after antigen exposure. IgM's effector functions are: activation of the classical pathway of complement; and as the antigen receptor of naive B lymphocytes.||None|
|IgG||γ||IgG is the most abundant antibody. IgG's effector functions are: opsonization of antigens for phagocytosis by macrophages and neutrophils; activation of the classical pathway of complement; antibody-dependent cell-mediated cytotoxicity (ADCC), mediated by natural killer cells and macrophages; and neonatal immunity, the transfer of maternal antibodies through placenta and gut.γ heavy chains contain four intrachain disulfide bonds and light chains contain two intrachain disulfide bonds; γ heavy chains and light chains are connected by interchain disulfide bonds. An entire IgG antibody is 150,000 daltons; each γ heavy chain is 50,000 daltons; and each light chain is 25,000 daltons.||IgG1, IgG2, IgG3, IgG4|
|IgA||α||IgA is present in secretions and protects the epithelium. IgA frequently polymerizes with IgM's J chain. In secretions, it also has a fourth chain (secretory component) which is a product of epithelial cells. Since IgA is secreted into the lumens of the gastrointestinal and respiratory tracts, IgA's effector function is to protect against pathogens which attack at the mucosal surface.||IgA1, IgA2|
|IgE||ε||IgE binds the FcR receptor on Mast cells, and allergies are initiated when the IgE-FcR complex binds an antigen. IgE is present in low concentrations. IgE's effector functions are: Mast cell degranulation, leading to immediate hypersensitivity (allergy); and antibody-dependent cell-mediated cytotoxicity (ADCC) involving eosinophils.||None|
|IgD||δ||IgD's effector function is as the membrane receptor of naive B cells, and is expressed by anergic B cells.||None|