Which immunoglobulin is primarily located on B-cell surfaces and functions as a receptor?

• A. IgD
• B. IgA
• C. IgG
• D. IgE

Answer: (A)

The main idea is that the B-cell receptor is a membrane-bound immunoglobulin that sits on the surface of B cells to recognize antigen. In mature naive B cells, this receptor is composed of membrane-bound immunoglobulins, primarily IgM and IgD, which act as the surface receptors that bind antigen and trigger activation signals. Among the options, the immunoglobulin that is primarily located on B-cell surfaces and functions as a receptor is IgD. It serves as part of the B-cell receptor complex, helping the B cell sense and respond to antigens. The other immunoglobulins—IgG, IgA, and IgE—are mainly secreted antibodies that carry out effector functions once produced by plasma cells (IgG for systemic defense and opsonization, IgA for mucosal immunity, IgE for allergic responses). They are not the principal membrane-bound receptors on B cells.