IgD

Immunoglobulin D (IgD) is one of the five major classes of antibodies produced by the adaptive immune system. In humans, IgD primarily serves as a membrane-bound receptor on naive B cells, where it commonly coexists with surface IgM as part of the B cell receptor. IgD is present in serum at much lower levels than other isotypes and is also found in some mucosal secretions.

IgD is encoded by the delta constant region (Cδ) of the heavy chain gene cluster. Naive B cells typically co-express membrane-bound IgD and IgM; these are generated through alternative RNA processing rather than extensive class switching. The Cδ region lies in the same heavy chain locus as other isotypes on chromosome 14, and cells can produce membrane-bound or secreted forms of IgD through differential splicing.

In terms of distribution, IgD is most prominent on the surface of mature, naïve B cells and contributes to the early stages of the humoral immune response. Secreted IgD is found in very small amounts in the blood, and its presence in mucosal tissues suggests a potential role in local immune surveillance. Compared with other immunoglobulins, the function of IgD is less clearly defined and remains an active area of research.

Functions attributed to IgD include participation in B cell development and signaling, as well as potential involvement in mucosal immunity. Some evidence indicates IgD can influence interactions with other immune cells and may help tailor responses to inhaled or encountered antigens, though the exact mechanisms are not fully established.

Clinically, markedly elevated IgD levels occur in hyper-IgD syndrome (a rare autoinflammatory condition associated with MVK deficiency). IgD deficiency is uncommon, and there is no IgD-focused therapy; assessments of IgD levels are typically part of broader immunoglobulin profiling.