Immunoglobulin isotype class switching is determined by:

Ans. is 'b' i.e., Constant region of heavy chain (Ref: Ananthanarayan, 9th/e, p. 99)* Immunoglobulin Class Switching: Initially, all B cells bound to an antigen carry IgM specific for that antigen and produce IgM in response to this antigen. Later, gene rearrangement generates antibodies of the same antigenic specificity but of different immunoglobulin classes. In class switching, the same assembled VH gene can sequentially associate with different CH genes, so that the immunoglobulin produced later (IgG, IgA, or IgE) has the same specificity as the original IgM but with different biologic characteristics. Class switching is dependent on cytokines released from T cells. Recently, IL-4, IL-5, IFN-g, and transforming growth factor-beta (TGF -b) have been shown to play a role in regulating Ig class switching.