The enzyme at the center of blood clotting—it converts fibrinogen into fibrin strands that form the structural mesh of a blood clot.Thrombin (Factor IIa) is a serine protease generated from prothrombin by the prothrombinase complex during coagulation. It converts fibrinogen to fibrin, activates platelets via PAR-1, amplifies coagulation through positive feedback (activating Factors V, VIII, XI), and regulates anticoagulation via thrombomodulin/protein C. Bivalirudin is a synthetic peptide (20 amino acids, derived from hirudin) that directly and reversibly inhibits thrombin—both clot-bound and circulating—providing anticoagulation during percutaneous coronary intervention.