Fibrin

Fibrin is a fibrous, insoluble protein that plays a central role in blood coagulation. It is formed when thrombin cleaves soluble plasma fibrinogen, a glycoprotein produced by the liver, removing fibrinopeptides A and B. The resulting fibrin monomers rapidly polymerize to form a loose fibrin network that, together with platelets, creates a hemostatic plug at sites of vascular injury.

The fibrin network is strengthened and stabilized by factor XIIIa, which crosslinks adjacent fibrin strands through covalent isopeptide bonds. This crosslinking converts the initial, soft clot into a stable structure that resists shear and proteolysis.

During fibrinolysis, plasmin degrades fibrin into soluble and insoluble fragments. The most recognizable degradation products are D-dimers, which reflect the breakdown of crosslinked fibrin and are used clinically to assess thrombotic activity.

Physiological role and clinical relevance: Fibrin forms the structural scaffold of a clot, trapping blood cells and reinforcing a platelet plug to limit bleeding. It also provides a provisional matrix for wound healing. Abnormal fibrin formation or degradation can contribute to disease: hypofibrinogenemia or dysfibrinogenemia leads to bleeding; excessive or improperly regulated fibrin formation can cause thrombosis; disseminated intravascular coagulation involves widespread fibrin deposition; liver disease or anticoagulant therapy can affect fibrinogen levels and fibrin formation.

Fibrin and fibrinogen are distinct but closely linked components of the coagulation cascade. Fibrinogen is a soluble 340-kilodalton glycoprotein circulating in blood, while fibrin refers to the insoluble polymer that forms the clot.