Fibrin, which is an insoluble protein, is formed from fibrinogen by action of thrombin and is an essential component for the clotting of blood; this elastic, whitish protein forms an interlacing fibrous network when it is used for coagulation and is chiefly responsible for the hardened appearance that a blood clot assumes. When fibrin is used to create a blood clot, it does so by forming a network that traps both red blood cells and platelets. Aside from blood clotting, fibrin is also used for platelet activation and signal transduction.
Fibrin does play a role in may diseases. An decrease in the production of fibrinogen, fibrin's inactive precursor, is sometimes caused by dysfunction of the liver; fibrinogen is, by definition, production of abnormal fibrinogen molecules with reduced activity. This type of absent, dysfunctional or reduced fibrinogen is most often seen in hemophiliacs. Thrombosis, which is when a vessel is blocked by agglutination of platelets, red blood cells and polymerized fibrin, is due to excessive production of fibrin due to activation of a coagulation cascade. This, in turn, can lead to ineffective generation of fibrin which causes hemorrhaging.
There also exists the possibility of being born with a hereditary fibrinogen abnormality. The gene for this abnormality is carried and found on the fourth chromosome. These abnormalities of fibrinogen include dysfibrinogenaemia, hypodysfibrogenaemia, afibrogenaemia and hypofibrinogenaemia, and are both qualitative and quantitative in nature.
When an injury that draws blood occurs, fibrin molecules quickly gather at the site of the wound. These molecules systematically form long strands of fibrin which effectively begin to catch all of the accumulating platelets. This action of trapped platelets builds up a spongy mass that starts to slowly harden until the open wound is sufficiently blocked enough to stop the flow of blood; this hardened mass is what is commonly known as a scab.