complement components), ii) blood cells (e. g. , platelets, monocytes, neutrophils), and iii) blood vessels (e. g. , endothelium, smooth muscles). Moreover, thrombin functions at all three levels of hemostasis, while the majority of cell types known are responsive to greater or lesser extents to thrombin. Thrombin furthermore functions both as a proteolytic enzyme with arginine- or lysine-directed specificities or as a nonenzymic activated protein 1 6 (hormone-like) involving receptor occupancy on certain cells.