Monoclonal B lymphocytes in multiple myeloma.

Individual specific antisera were raised against the monoclonal serum component from six IgG myeloma patients. Idiotypic (Id) determinants were identified in direct or indirect immunofluorescence (IFL) on 0.5-44% B lymphocytes from five patients with active disease. Id determinants were not detected on B lymphocytes from one IgG-k myeloma patient with an imbalance in the expression of upsilon heavy chains and kappa light chains. Nor were Id determinants detected on E-rosetting cells. Id structures were usually present together with upsilon chains on monoclonal B lymphocytes, some of which also carried mu and/or delta chains. Blood cells with lymphoplasmacytoid appearance were abundant in two patients. These cells carried IgG with Id determinants both in the cytoplasm and on the surface. Fc receptors were present on most cells carrying surface Id structures but were rare on cells with cytoplasmic monoclonal characteristics. The results further support the hypothesis of multiple myeloma as a differentiating B-lymphocyte tumour.