What is somatic hypermutation and class switch recombination, and in which cells do they occur?

• A. Somatic hypermutation alters constant regions of antibodies; class switch recombination increases affinity.
• B. Somatic hypermutation introduces mutations in antibody variable regions to increase affinity; class switch recombination changes antibody isotype; both occur in activated B cells.
• C. Both occur in T cells.
• D. Neither occurs in humans.

Answer: (B)

Somatic hypermutation and class switch recombination modify antibody genes in B cells during an immune response. Somatic hypermutation introduces point mutations in the variable regions of antibody genes, generating B cells with a spectrum of affinities for the antigen; those with higher affinity are selected in germinal centers, increasing overall antibody affinity. Class switch recombination changes the constant region of the heavy chain, switching the antibody isotype (for example from IgM to IgG, IgA, or IgE) without altering antigen specificity, thereby altering how the antibody interacts with other parts of the immune system. Both processes occur in activated B cells within germinal centers of secondary lymphoid organs and require activation-induced cytidine deaminase (AID). They do not occur in T cells, and they occur in humans. This matches the description that somatic hypermutation introduces mutations in variable regions to increase affinity, and class switch recombination changes antibody isotype, with both happening in activated B cells.