immunoglobulin D (IgD), one of the five classes of antibodies produced by the body. It is found in small amounts in serum tissue. Although its precise function is not known, IgD increases in quantity during allergic reactions to milk, insulin, penicillin, and various toxins. The normal concentration of IgD in serum is 0.5 to 3 mg/dL. Compare immunoglobulin A, immunoglobulin E, immunoglobulin G, immunoglobulin M.
