Which heavy chain is unique to the IgD immunoglobulin class?

• A. Gamma
• B. Mu
• C. Delta
• D. Epsilon

Answer: (C)

Antibody class is defined by the heavy-chain constant region, which differentiates the isotypes. Each class uses a distinct constant region: mu for IgM, delta for IgD, gamma for IgG, alpha for IgA, and epsilon for IgE. IgD specifically uses the delta heavy chain, whereas the others use their respective heavy chains. In naïve B cells, IgM and IgD can be expressed from the same VDJ rearrangement because the same variable region combines with either the mu or delta constant region through RNA processing, giving two surface receptors with the same antigen specificity but different effector properties. Because the delta heavy chain is reserved for IgD, it is the one unique to this class.