FIGURE 23-20 Class-switch recombination in the immunoglobulin heavy-chain locus. Class-switch recombination involves switch sites, which are repetitive sequences (colored circles) upstream of each of the heavy-chain constant-region genes. Recombination requires activation-induced deaminase (AID) as well as cytokines (e.g., IL-4) produced by certain helper T cells. Recombination eliminates the segment of DNA between the switch site upstream of µ exons and the constant region to which switching occurs. Class switching generates antibody molecules with the same specificity for antigen as that of the IgM-bearing B cell that mounted the original response, but with different heavy-chain constant regions and therefore different effector functions.