IgD is the major antigen receptor isotype on the surface of most peripheral B cells, where it is co expressed with IgM. It is a unique immunoglobulin with molecular weight of 185,000 Dts. It represents about 0.25% of the total serum immunoglobulins (average serum level is 0.03mg/ml) and has a half-life of 2-3 days, similar to that of IgE.
Its synthesis rate is at least 10 times lower than that of IgA, IgM, and IgG. The IgD molecule has a long “hinge” region that appears to make the fragment very susceptible to proteolytic degradation with production of Fab and Fc fragments and also renders the molecule flexible, thus enhancing antigen binding.
Carbohydrates occupy about 12% of total molecular weight. The sedimentation coefficient of IgD is 7S and rate of synthesis is 0.4mg/kg body weight/day.
Mature B cells express a membrane bound immunoglobulin and it is thought to function in the activation of B cells after binding with antigens. Serum IgD and membrane-bound IgD are antigenically similar, but they differ in susceptibility to proteolysis by plasmin (A proteolytic enzyme that is formed from plasminogen in blood plasma dissolves the fibrin in blood clots.).
ADVERTISEMENTS:
Instead of being secreted in large amounts, after antigen binding to the B-cell receptor, the secreted form of 0 chains is shut off. The B cells enter the pathways of plasma cells (which do not have IgD except for IgD-secreting plasma cells, where the p gene is deleted) or of memory cells in secondary lymphoid organs.