Platelet glycoprotein Ib (GpIb), a receptor for von Willebrand's factor (vWF), was studied by way of fluorescence flow cytometry. Using a sandwich staining technique, GpIb was identified by a monoclonal antibody (6D1) directed against an epitope close to the vWF binding site. Platelets from normal individuals were symmetrically distributed with respect to GpIb content. Treatment of washed platelets with plasmin resulted in progressive loss of GpIb as measured by fluorescence flow cytometry and by loss of agglutination response when combined with ristocetin in the presence of vWF. In mixing experiments with GpIb-deficient and normal platelets, it was possible to detect a subpopulation of deficient cells comprising 2% of the total population. Streptokinase treatment of platelet-rich plasma caused loss of the agglutination response to ristocetin and the emergence of a population of GpIb-deficient platelets. Fluorescence flow cytometry appears to be an important new technique by which to study platelet surface receptors.

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