Isotype (immunology)
In immunology, antibodies (immunoglobulins (Ig)) are classified into several types called isotypes or classes. The variable (V) regions near the tip of the antibody can differ from molecule to molecule in countless ways, allowing it to specifically target an antigen (or more exactly, an epitope). In contrast, the constant (C) regions only occur in a few variants, which define the antibody's class. Antibodies of different classes activate distinct effector mechanisms in response to an antigen (triggering different elements of the innate immune system). They appear at different stages of an immune response, differ in structural features, and in their location around the body. Isotype expression reflects the maturation stage of a B cell. Naive B cells express IgM and IgD isotypes with unmutated variable genes, which are produced from the same initial transcript following alternative splicing. Expression of other antibody isotypes (in humans: IgG, IgA, and IgE) occurs via a process of class switching after antigen exposure. Class switching is mediated by the enzyme AID (activation-induced cytidine deaminase) and occurs after the B cell binds an antigen through its B cell receptor. Class-switching usually requires interaction with a T helper cell. In humans, there are five heavy chain isotypes α,δ,γ,ε,μ, corresponding to five antibody isotypes: α – IgA, further divided into subclasses IgA1 and IgA2 δ – IgD γ – IgG, further divided into subclasses IgG1 to IgG4 ε – IgE μ – IgM There are also two light chain isotypes κ and λ; however, there is no significant difference in function between the two. Thus an antibody isotype is determined by the constant regions of the heavy chains only. IgM is first expressed as a monomer on the surface of immature B cells. Upon antigenic stimulation, IgM+ B cells secrete pentameric IgM antibody formed by five Ig monomers are linked via disulfide bonds. The pentamer also contains a polypeptide J-chain, which links two of the monomers and facilitates secretion at mucosal surfaces.
