What role does IgD primarily serve in the immune system?

IgD primarily serves as a receptor on B cells, playing a critical role in the activation of these immune cells. When B cells develop in the bone marrow, they express IgD on their surface, which helps them recognize specific antigens. This interaction is essential for the differentiation and proliferation of B cells following antigen exposure, leading to the production of antibodies. While IgD is present in small amounts compared to other immunoglobulins, its function as a B cell receptor is vital for initiating the adaptive immune response.

The other roles suggested in the choices, while important within the immune system, do not accurately describe the primary function of IgD. For example, the primary antibody response is typically associated with IgM and later IgG. The main defense against parasites is generally mediated by IgE, which is important for allergic reactions and responses to helminths. Similarly, while IgD is present in the context of allergic responses, it does not directly mediate those responses, as that role is mainly fulfilled by IgE. Thus, the emphasis on IgD's role as a receptor on B cells reflects its crucial contribution to the immune system's ability to respond to pathogens effectively.