The protein that forms the structural mesh of blood clots—it polymerizes into long strands that trap blood cells and seal wounds.Fibrin is an insoluble protein polymerized from fibrinogen by thrombin-mediated cleavage of fibrinopeptides A and B. Fibrin monomers spontaneously polymerize into protofibrils, then are cross-linked by Factor XIIIa into a stable clot matrix. Fibrin clots are eventually dissolved by plasmin (fibrinolysis). In peptide cardiovascular therapeutics, controlling fibrin formation is the target: bivalirudin inhibits thrombin to reduce fibrin generation; eptifibatide blocks platelet aggregation that contributes to clot formation.