The proteins of the mammalian blood coagulation system,a group of proteins and regulators whose major job is to stop bleeding,exist in the plasma vascular compartment and have opportunity to interact with multiple other proteins.The article by Tang et al.presents the novel observation that transferrin,an iron transport protein that exists in plasma in concentration excess to all the blood coagulation proteins,promotes blood coagulation by increasing thrombin and factor Xlla activities and inhibiting antithrombin.