[Thrombocytopenia in Waldenstrom's disease with specific antithrombocytic properties of the IgM paraprotein].

The serum of a patient suffering from thrombocytopenia associated with Waldenström's macroglobulinemia was found to contain an IgM paraprotein with unusual characteristics. Following isolation of this IgM paraprotein, dilutions up to 0.01 mg/ml caused immunofluorescence of thrombocytes from man, sheep, rabbit, and horse. In control studies with isolated IgM paraprotein from another non-thrombocytopenic patient with Waldenström's disease, positive immunofluorescence could be elicited only when a 1000-fold greater concentration of the paraprotein was used. The IgM paraprotein from our thrombocytopenic patient and that from the control patient showed comparable differences in immunofluorescence of thrombocytes and megacaryocytes when bone marrow smears were used for incubation. Further studies following papain cleavage of the IgM paraprotein with antithrombocytic activity showed that the combining activity was located in the Fab fragment. This observation characterizes the IgM paraprotein from our patient as an antibody directed to a substance in human and animal thrombocytes. Paraproteins with antibody-like activity for thrombocytes have not been identified previously; the present observation suggests that this mechanism should be considered a possible cause of thrombocytopenia associated with macroglobulinemia.