Low molecular weight IgM in B cell lymphoproliferative disorders.

Circulating low molecular weight (LMW) IgM was demonstrated in five of 38 patients with B cell lymphoproliferative disorders. These five patients all had malignant disease and could be subdivided into two groups. In the first group were three patients, each with an associated serum IgM paraprotein; two had Waldenström's macroglobulinemia, and one lymphocytic lymphoma. The two patients of the second group did not have IgM paraproteins; one had lymphocytic lymphoma and one chronic lymphocytic leukemia. Both these patients also had acquired C1 esterase inhibitor deficiency, a previously recognised association with circulating LMW IgM. None of the 16 patients with benign IgM macroglobulinemia had circulating LMW IgM. In those positive sera with LMW IgM this moiety contributed between 10.5% and 37.5% of the total IgM. There was no apparent association between LMW IgM and total IgM levels, kappa/lambda typing or the presence of Bence Jones proteinuria, but rheumatoid factor, immune complexes and cryoglobulins occurred in many of the sera which contained LMW IgM. Pokeweed mitogen stimulated peripheral blood mononuclear cells from two patients with circulating LMW IgM secreted considerable quantities of this moiety in vitro but this did not occur in two patients with benign IgM macroglobulinemia. We conclude that LMW IgM is found in the malignant but not the benign forms of B cell lymphoproliferative disorders and is frequently associated with other serological abnormalities. The basic abnormality causing defective IgM polymerisation in these disorders is obscure.