PLATELET BINDING OF IGG FROM PATIENTS WITH HEPARIN-INDUCED THROMBOCYTOPENIA

Although heparin induces immune-mediated thrombocytopenia, it has been difficult to demonstrate heparin specificity of the putative immunogl obin. Recently, however, a body of data has indicated that platelet fa ctor 4 (PF4) is required for heparin-induced thrombocytopenia (HIT) an tibody to bind to heparin. Using viable platelets in a physiologic buf fer, we have now documented specific and reversible platelet binding o f iodine 125-labeled IgG from 5 patients with HIT and binding of I-125 -labeled F(ab')(2) from 2 of them. The binding requires the presence o f both heparin and PF4 in a molar ratio of similar to 1:1. We have als o shown that platelet activation increases HIT antibody binding. Our d ata suggest that the F(ab')(2) domain of HIT antibody binds to heparin -PF4 complexes that accumulate on the platelet surface when equimolar concentrations of heparin and PF4 are present.