Which immunoglobulin controls humoral immune responses by regulating B-cell secretion?

• A. IgD
• B. IgA
• C. IgG
• D. IgE

Answer: (A)

The key idea is that humoral immunity is controlled at the B cell level by the B cell receptor, which is formed by membrane-bound immunoglobulins on the surface of mature naive B cells. IgD, along with IgM, is expressed on these B cells and functions as part of the receptor complex that senses antigen and modulates B cell signaling. By tuning the activation threshold and guiding when B cells become activated to secrete antibodies, IgD helps regulate the magnitude and timing of the humoral response. The other immunoglobulins—IgG, IgA, and IgE—chiefly serve as effector molecules after B cell activation and class switching (neutralization, opsonization, mucosal protection, allergic and anti-parasitic responses), rather than regulating B-cell secretion themselves. Thus IgD is the one best described as controlling humoral responses by regulating B-cell secretion.