What is antibody class switching and what prompts it?

• A. Replacement of antibody isotype by recombination in B cells; prompts include helper T cell signals and cytokines.
• B. Somatic hypermutation altering antigen-binding specificity.
• C. Clonal expansion of B cells without changing isotype.
• D. Receptor editing in immature B cells.

Answer: (B)

Class switch recombination changes the heavy-chain constant region of an antibody, so a B cell can produce a different isotype (for example IgM to IgG, IgA, or IgE) while keeping the same antigen-binding specificity. This shifts the antibody’s effector functions—such as how it opsonizes, activates complement, or operates at mucosal surfaces—without altering what it recognizes. The switch is prompted by helper T cell signals: CD40 ligand on activated T cells binds CD40 on B cells, providing essential costimulation, and cytokines from these T cells direct which isotype will be produced (IL-4 can favor IgE and certain IgG subtypes; TGF-β promotes IgA; IFN-γ promotes other IgG subclasses). Mechanistically, CSR involves recombination between switch regions in the heavy-chain constant-region genes and is facilitated by the enzyme AID, occurring during germinal center reactions. This is distinct from somatic hypermutation, which mutates the variable region to change affinity, not the isotype; clonal expansion and receptor editing are separate B cell processes.