Figure 7.

Class switch recombination at the immunoglobulin heavy chain locus. Unlike V(D)J recombination, which occurs at all three Ig loci (heavy chain, κ chain, and λ chain loci) and all three TCR loci (α/δ, β, and γ loci), class switch recombination occurs only at the IgH locus. The right-angle arrows represent internal promoters called sterile transcript promoters. The transcripts from these do not encode protein because of stop codons. This suggests that transcription may be important for the mechanism of class switch recombination. The ovals represent the G-rich repetitive class switch sequences. Recombination occurs between switch sequences. Not all of the sterile transcript promoters are active at any one time. The cytokine stimulation of B cells in the peripheral lymphoid tissues determine which promoters are active.