IgD (Molecular Biology)

IgD is an immunoglobulin that is present on the surface of mature B cells. Its circulating concentration is in the range of 40-400 |ig/mL, which does not contribute significantly to the overall amount of secreted Ig (compare with the 10 mg/mL range for IgG). IgD exists as only one isotype and is exclusively a d 2k2 or d 2l2 monomer. The d chain in humans has three constant domains, whereas there are only two in murine species, in which Cd2 is replaced by an unusually long hinge region. The main characteristic of IgD is to be essentially coexpressed at the surface of the same B cell with IgM. This does not contradict the clonal selection theory, because the two B-cell receptors express the same variable regions and hence have the same antibody specificity. This is because the switch region that is located at the 5′ position of all C gene isotypes is absent from the Cd locus. Coexpression of m and d chains results from alternative splicing, thus leading to both IgM and IgD that are associated to same k or l chains within a given B cell. Occasionally, clones expressing solely IgD without IgM can be encountered. This is most apparent in rare myeloma cells that are IgD producers. In that case, the absence of IgM probably results from a deletion of the Cm genes at the genomic level. This seems also to be the case of a recent observation of B cells detected in human tonsils that express exclusively IgD and have surprisingly accumulated an unusually large number of somatic mutations. Despite their very peculiar specialization as surface Ig, the role of IgD is still very poorly understood, although some arguments point to their possible role in B-cell induction of tolerance.

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