Short Description: IgD (IgD) is a monomeric antibody isotype that is expressed in the plasma membranes of immature B-lymphocytes. IgD is also produced in a secreted form that is found in small amounts in blood serum. Secreted IgD is made up of two heavy chains of the delta class, and two light chains. IgD's function has long been a conundrum and is still incompletely understood. IgD is present in species ranging from fish to human (probably with the exception of birds), indicating that IgD has been preserved since its ancient origins which suggests that IgD has important immunological functions. More information related to gene IgD.