What is fibrin?

   Fibrin is a protein that is formed during the clotting of blood. Fibrin, which is a whitish, elastic substance composed of many tiny strands, is not normally found in the blood. In the process of clotting, however, a network of fibrin strands is formed from fibrinogen, another protein, which is normally present in the blood. Blood cells then become entangled in the strands of fibrin. The mixture of fibrin and blood cells is called a blood clot. With time, the fibrin strands contract and shorten, causing the clot to become more dense.