What characteristic differentiates IgD from the other immunoglobulin classes?

The distinguishing characteristic of IgD among the immunoglobulin classes is that it is predominantly located on the surfaces of B cells. This feature allows IgD to play a crucial role in the initiation and regulation of B cell activation and differentiation. When B cells encounter antigens, IgD can facilitate the signaling pathways that lead to B cell proliferation and the subsequent production of antibodies.

In contrast to other immunoglobulin classes, IgD's primary function is associated with its presence on B cell membranes rather than in serum or secretions. For instance, IgA is mainly found in secretions like saliva and mucus, while IgE is associated with allergic reactions, and IgG is the most abundant antibody in serum, playing significant roles in immune responses. Thus, IgD's unique positioning on B cells sets it apart from these other classes, emphasizing its role in the early stages of the adaptive immune response.