Measurement of IgD by a double antibody radioimmunoassay: demonstration of an apparent trimodal distribution of IgD levels in normal human sera

A double antibody radioimmunoassay (RIA) was developed to measure IgD in serum and secretions. One IgD myeloma protein was used as radiolabeled antigen and standard with antiserum to a second IgD myeloma protein. The IgD standard and normal sera yielded parallel inhibition curves in the RIA and inhibition was produced by IgD and not by any of the other immunoglobulins. The assay had a lower limit of sensitivity of 0.01 International Unit (I.U.)/ml and modifications increased the sensitivity to 0.0008 I.U./ml. Measurable IgD levels were found in all 112 normal adult sera assayed (geometric mean 13.0 I.U./ml, arithmetic mean 30.1 I.U./ml, median 14.8 I.U./ml, range 0.10 to 202 I.U./ml, 95 percent of values between 0.19 and 156 I.U./ml. The distribution of IgD in the 112 normal sera appeared trimodal with modes at approximately 0.25 I.U./ml, 5 I.U./ml, and 35 I.U./ml. IgD was measurable in nasal and bronchial washes and human milk, but could not be detected in parotid fluid