The protein that helps blood to clot is fibrin. It forms a tough, insoluble mesh that strengthens the initial platelet plug and seals a damaged blood vessel.
Fibrin is produced from fibrinogen, a soluble protein circulating in blood plasma. When a vessel is injured, the enzyme thrombin removes small pieces from fibrinogen, allowing the remaining fibrin molecules to join into long strands. These strands spread across the wound and trap blood cells and platelets.
The enzyme factor XIIIa then cross-links the fibrin strands, making the clot stronger and more stable. Later, the body breaks the clot down through fibrinolysis; plasmin cuts the fibrin network into fragments, including D-dimer.
A common mix-up is fibrinogen: it is the precursor, while fibrin is the actual mesh in the finished clot. Hemoglobin carries oxygen inside red blood cells, insulin regulates blood glucose, and “enzymes” is a broad category rather than one specific clotting protein. Fibrin was described in studies associated with Marcello Malpighi in 1666.