Association of myeloma protein with IgG and its poor reactivity against anti-human .LAMBDA. antibodies. A case of IgD (.LAMBDA.)-type multiple myeloma.

We encountered a case of IgD-type multiple myeloma in which we could not identify the type of light (L) chains of the IgD M-protein in the patient's serum by conventional immunoelectrophoresis or immunofixation electrophoresis. To clarify the cause of this failure, the immunochemical and structural properties of IgD in the patient's serum were characterized. The immunochemical reactivity of IgD M-protein against anti-λ anti- bodies was extremely low and the IgD-IgG complex was present in the patient's serum. The complex was dissociated in the presence of SDS, indicating that the association was formed via non-covalent binding. In a reassociation test, IgG prepared from normal subjects did not form a complex with the patient's IgD pretreated with a reducing agent, but formed a complex with IgD pretreated with papain. These results suggest that IgG formed a complex with the Fab of the patient's IgD.