Monoclonal IgM with lupus anticoagulant activity in a case of Waldenström's macroglobulinaemia.

Strong lupus anticoagulant activity was detected in the plasma of a 73-yr-old woman with Waldenström's macroglobulinaemia. Purified patient IgM caused significant prolongation of the APTT in concentrations as low as 30 micrograms/ml, and its activity was inhibited by intact or (more effectively) by frozen-thawed blood platelets. Employing varying concentrations of cephalin in the APTT assay, it was demonstrated that the macroglobulin had a direct cephalin-neutralizing effect. When tested in an ELISA system using phospholipid-coated microtiter wells, the IgM was shown to possess antibody activity against cephalin. With purified phospholipids as antigens, the protein was found to display antibody activity against phosphatidyl serine and phosphatidyl ethanolamine, which are main constituents of cephalin. These observations were confirmed in absorption experiments.