Class-switch recombination and somatic hypermutation. Through class-switch recombination (left), the originally expressed Cμ and Cδ heavy chain constant region genes (coding for IgM and IgD, respectively) are replaced by one of the downstream C_H genes. The process of somatic hypermutation (right) introduces mainly point mutations (denoted by “x”) at very high rates, specifically into the Ig variable region genes. Both processes strictly depend on AID activity but can occur independently in B cells.