Actions and Interactions of Antithrombin and Heparin

THE coagulation of blood occurs via two pathways in which activation of specific plasma proteins initiates a cascade of reactions that ultimately lead to a fibrin clot. In the first pathway, injury to the endothelial cell layer exposes collagen1 , 2 or other subintimal components that activate Factor XII. Activated Factor XII activates Factor XI,3 which, in turn, activates Factor IX.4 Together with platelets and a cofactor, Factor VIII, activated Factor IX then activates Factor X.4 In the second pathway, subtle alterations in the endothelium expose tissue factor, which activates Factor VII,5 which in turn directly activates Factor X.5 Once activated by . . .