The Endogenous Lectin of Human Platelets is

hemagglutination activity (enhanced lectin activity) not expressed by nonactivated platelets, and this agglutinin mediates the direct platelet-platelet interactions that cause the aggregation of thrombinactivated platelets. The enhanced agglutinin activity of thrombin-activated platelets is composed of two components: a plasma membrane-bound component and a soluble component.3 The expression of both forms of the agglutinin activity is dependent on secretion.”3 Accordingly, washed platelets exposed to fibrinogen and adenosine diphosphate (ADP) in Tyrode buffer, in the absence of stirring, lack enhanced hemagglutination activity (unpublished data). The soluble, but not the membrane-bound, agglutinin activity is dependent on fibrin formation.3 The function of both forms of the agglutinin activity is inhibited by the same compounds that inhibit the aggregation of thrombin-activated platelets. In this article, platelets from individuals with congenital bleeding disorders that have selective effects were studied in an attempt to correlate their associated platelet agglutinin deficiencies, if any, with their abnormal behavior in aggregation. We describe the platelet-bound and platelet-free (soluble) agglutinin activities of activated washed platelets from patients with the bleeding disorders, congenital afibrinogenemia,4 Glanzmann’s thrombasthenia,56 gray platelet