An unusual case of a plasma cell neoplasm with an IgG3lambda myeloma and a gamma3 heavy chain disease protein.

A unique case of gamma3 heavy chain disease with two related serum proteins is reported. One molecule appears to be an IgG3lambda myeloma protein. The second molecule is a dimer of a shortened gamma3 heavy chain that has an unblocked amino terminus and lacks the VH and CH1 domains. Its probable origin as a synthetic product is discussed. The clinical and pathologic features of this patient resemble those of other patients with gamma heavy chain disease more than those of patients with multiple myeloma. It seems likely that the heavy chain disease protein is the result of a mutational event in the malignant clone originally producing the myeloma protein.