Fibrin: The Protein Mesh That Stops Bleeding

Fibrin is the insoluble protein network that forms the structural basis of a blood clot.

Formation

1. Vessel injury exposes tissue factor.
2. Coagulation cascade activates thrombin.
3. Thrombin cleaves fibrinogen → fibrin monomers.
4. Monomers polymerize into mesh.
5. Factor XIII cross-links fibrin → stable clot.

Function

The fibrin mesh traps platelets and red cells, sealing the breach. Plasmin later degrades fibrin (fibrinolysis) during healing.

Clinical Relevance

• D-dimer = fibrin degradation product; elevated in thrombosis/DIC.
• Fibrinogen deficiency → bleeding.
• Thrombolytics (tPA) activate plasminogen to dissolve clots.