During class switch recombination, which statement describes what happens to the heavy chain?

• A. Changes constant region but preserves antigen specificity
• B. Changes variable region
• C. Eliminates heavy chain
• D. Switches to light chain

Answer: (D)

During class switch recombination, the heavy chain changes its constant region to become a different antibody isotype (for example IgM to IgG, IgA, or IgE), while the antigen-binding variable region stays the same. The variable region, encoded by the VDJ rearrangement, determines what antigen the antibody binds, so preserving it means the antibody retains the same specificity. What CSR does is swap the constant region to alter effector functions—how the antibody acts in the immune response—without changing which antigen it recognizes. It does not alter the variable region, it does not eliminate the heavy chain, and it does not switch to the light chain.