- von Willebrand factor synthesized by endothelial cells from a patient with type IIB von Willebrand disease supports platelet adhesion normally but has an increased affinity for platelets. de Groot, P.G., Federici, A.B., de Boer, H.C., d'Alessio, P., Mannucci, P.M., Sixma, J.J. Proc. Natl. Acad. Sci. U.S.A. (1989)