Immunoglobulin D (IgD) is a unique immunoglobulin found in low concentrations in the serum, comprising only about 0.2% of the total immunoglobulin in the serum. It is the second Ig isotype to be synthesized by a B cell, starting to be expressed when the B call leaves the bone marrow to populate lymphoid tissues.  IgD coexists with IgM on the surface of B lymphocytes where it acts as an antigen recognition receptor. IgD is made up of four heavy chain (δ) domains. The heavy chains are covalently linked by only one disulfide bridge.