fibrin

Fibrinogen (factor I), a soluble plasma glycoprotein produced by the liver, is converted to fibrin monomers by thrombin (factor IIa), which cleaves fibrinopeptides A and B. Fibrin monomers polymerize spontaneously into soft, loosely associated protofibrils. Factor XIIIa (activated by thrombin) then catalyzes covalent cross-linking between fibrin monomers, converting the soft clot into an insoluble, mechanically stable fibrin mesh that traps platelets and red blood cells to form a definitive clot (secondary hemostasis).

Clinical relevance: - Coagulation cascade: Fibrin formation is the terminal step of both intrinsic and extrinsic pathways converging at the common pathway (factor X activation → prothrombin to thrombin → fibrinogen to fibrin). - PT/INR and PTT tests assess different portions of this cascade but ultimately depend on fibrin formation as the endpoint. - Fibrinolysis: Plasmin degrades fibrin into fibrin degradation products, including D-dimer, which is elevated in conditions with increased clot formation/breakdown (DVT, PE, DIC, post-surgery). - Disseminated intravascular coagulation (DIC): widespread fibrin deposition in microvasculature with consumption of clotting factors, leading to both thrombosis and hemorrhage. - Fibrin plays a role in wound healing as a provisional matrix for tissue repair and inflammatory cell migration. - Therapeutic relevance: tPA (tissue plasminogen activator) promotes fibrinolysis by converting plasminogen to plasmin, used in treating acute ischemic stroke, PE, and MI. - Virchow's triad and clot formation are tested concepts linking fibrin to thrombosis risk factors. - Fibrin glue is used surgically for hemostasis and wound closure.

High-yield boards point: Differentiate fibrin (structural clot protein) from fibrinogen (precursor) and understand the coagulation cascade's convergence point at fibrin formation, as well as the clinical utility of D-dimer testing.