Willebrand Factor and Ristocetin I. MECHANISM OF RISTOCETIN‐INDUCED PLATELET AGGREGATION

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Abstract
Summary. Ristocetin induces aggregation of normal platelet‐rich plasma over a wide range of concentrations. Low doses induce a biphasic response of which the first wave is not mediated by ADP and proceeds without the initial platelet shape change. The absence of aggregation in von Willebrand's disease results from the lack of a component, Willebrand factor, which is eluted together with platelets, factor VIII activity and Willebrand (or factor VIII related) antigen in the void volume fraction when platelet‐rich plasma is gel filtered (Sepharose 2B). Twice washed normal platelets aggregate to ristocetin whereas four times washed platelets only aggregate when low volumes of normal haemophilia A plasma or serum are added. The interaction of ristocetin with platelets and plasmatic components has been investigated. A mechanism is proposed for ristocetin‐induced aggregation.