Type IIB von Willebrand factor with normal sialic acid content induces platelet aggregation in the absence of ristocetin. Role of platelet activation, fibrinogen, and two distinct membrane receptors - De, M arco L; Mazzuccato, M; Del, B en MG; Budde, U; Federici, A B; Girolami, A; Ruggeri, Z M (J Clin Invest, 1987)