Which class of antibody, when in monomer form, serves as antigen receptors on B cells?

• A. IgG
• B. IgM
• C. IgD
• D. IgE

Answer: (B)

The correct answer is IgD. This antibody class is distinct because it primarily functions as an antigen receptor on the surface of B cells in its monomer form. B cells, upon encountering a specific antigen, can bind to it through the IgD receptors. This binding event is crucial for triggering B cell activation, leading to clonal expansion and the differentiation of B cells into plasma cells and memory cells.

While IgG is the most abundant antibody in circulation and has various roles including neutralization of pathogens and opsonization, it is primarily involved in the immune response after B cell activation. IgM, although important for the initial immune response and often found as a pentamer in circulation, does not serve as the primary antigen receptor on B cells. IgE is mainly associated with allergic responses and protection against parasitic infections but also does not fulfill the role of antigen receptors on B cells. Thus, IgD is the correct antibody class that acts as an antigen receptor in the context described.