Immunoglobulin D (IgD) is one of the five classes of antibodies, primarily found as a monomer with a molecular weight of approximately 184 kDa and a half-life of 2.8 days. It is predominantly expressed on the surface of mature B cells, where it serves as an antigen receptor alongside IgM, playing a role in B cell activation, proliferation, and differentiation. IgD is also present in mucosal secretions and blood, and it can bind to basophils and mast cells, potentially activating them to release antimicrobial factors and contributing to mucosal immunity. Although its exact function remains enigmatic, IgD is believed to enhance humoral immune responses, participate in B cell memory, and possibly interfere with viral replication. Additionally, IgD may have a role in allergic reactions, though this area requires further investigation.