Which process enables B cells to produce different antibody isotypes (e.g., IgG, IgA, IgE) after initial IgM production?

• A. Somatic hypermutation
• B. Clonal deletion
• C. Antigen presentation
• D. Class switch recombination

Answer: (D)

Class switch recombination is the process that lets B cells change the heavy-chain constant region from IgM to IgG, IgA, or IgE while keeping the same antigen-binding specificity. After activation, B cells receive signals from helper T cells (such as CD40L and specific cytokines) that initiate recombination at the switch regions upstream of the constant region genes. This DNA rearrangement replaces the μ constant region with γ, α, or ε, producing antibodies with different effector functions suited to different contexts (IgG in serum, IgA at mucosal surfaces, IgE in allergy and parasite defense). Somatic hypermutation, by contrast, alters the variable region to increase affinity rather than change the isotype, while clonal deletion and antigen presentation serve different roles in tolerance and antigen processing.