Fibrinogen Competes With von Willebrand Factor for Binding to the Glycoprotein lib/Illa Complex When Platelets Are Stimulated With Thrombin

Two monoclonal antibodies-one that blocks ristocetininduced platelet binding of von Willebrand factor to glycoprotein lb and one that blocks adenosine diphosphateinduced binding of fibrinogen to the glycoprotein llb/Illa complex-were used to assess the binding site(s) for von Willebrand factor when platelets are stimulated with thrombin or adenosine diphosphate (ADP). Neither agonist induced binding of von Willebrand factor to glycoprotein lb. ADP and thrombin induced von Willebrand factor binding exclusively to the glycoprotein lIb/Illa complex. The results