T helper cell
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.