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 collagen^{1 , 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 . . .