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The immunoglobulin that accounts for less than 1% of the immunoglobulin pool and is primarily found on B-lymphocytes is IgD. This immunoglobulin plays a crucial role in the activation and the function of B cells. While IgD is present in very small quantities compared to other immunoglobulins, like IgG, IgM, or IgE, it serves as a B cell receptor (BCR) on the surface of immature B lymphocytes. This surface expression is essential for the development and maturation of B cells in the immune response.

IgD's exact function is not fully understood compared to other immunoglobulins, but it is believed to be involved in the initiation and regulation of B cell activation upon antigen encounter. Its low presence in the serum, in contrast to more abundant antibodies, highlights its specialized role in B cell biology rather than in systemic immune responses.