Immunoglobulins are made in several distinct isotypes or classes—IgM, IgG, IgD, IgA, and IgE—each of which has a distinct heavy-chain C region encoded by a distinct C-region gene. The isotype of an antibody determines the effector mechanisms that it can engage on binding antigen. The different heavy-chain C regions are encoded in exons 3′ to the V(D)J rearrangement site. This allows the same antibody heavy-chain V region to link up with different heavy-chain C-region isotypes as a result of somatic recombination.