IgDs

Immunoglobulin D (IgD) is one of the five major classes of immunoglobulins found in jawed vertebrates. In humans, IgD is primarily expressed on the surface of naive B lymphocytes as part of the B cell receptor (BCR) together with membrane-bound IgM; circulating IgD exists at relatively low levels in serum, and its precise functions are less well defined.

IgD is encoded by the delta heavy-chain (IGHD) gene within the immunoglobulin heavy-chain locus. On B cells, IgD is produced by alternative splicing of the heavy-chain transcript to generate a membrane-bound form; in most species, secreted IgD is present only in small amounts. The IgD constant region is known as the delta (δ) region, and the gene is located adjacent to the IgM constant region on chromosome 14 in humans.

Naive B cells typically co-express IgM and IgD on their surface, and signaling through the IgD-containing BCR contributes to B cell development and activation. In addition to its role as part of the BCR, secreted IgD is implicated in mucosal immunity and may participate in modulating immune responses, including potential interactions with basophils that influence cytokine production and inflammatory processes. The exact mechanisms and contexts of IgD’s actions are still an area of active research.

Clinical and research notes: IgD deficiency is rare and not widely documented. Some conditions, such as hyper-IgD syndrome (HIDS), involve markedly elevated serum IgD and recurrent fever episodes, highlighting a pathological link between IgD levels and immune regulation. Routine clinical testing does not typically emphasize IgD measurement, but it remains a topic of interest in studies of B cell biology and mucosal immunity.