Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed from fibrinogen by the protease thrombin, and is then polymerised to form a "mesh" that forms a hemostatic plug or clot (in conjunction with platelets) over a wound site.
Role in disease 
Excessive generation of fibrin due to activation of the coagulation cascade leads to thrombosis, the block of a vessel by an agglutination of red blood cells, platelets, polymerized fibrin and other components. Ineffective generation or premature lysis of fibrin predisposes to hemorrhage.
Dysfunction or disease of the liver can lead to a decrease in the production of fibrin's inactive precursor, fibrinogen, or to the production of abnormal fibrinogen molecules with reduced activity (dysfibrinogenaemia). Hereditary abnormalities of fibrinogen (the gene is carried on chromosome 4) are of both quantitative and qualitative in nature and include afibrinogenaemia, hypofibrinogenaemia, dysfibrinogenaemia, and hypodysfibrinogenaemia.
Consequences of reduced, absent, or dysfunctional fibrin is likely to render patients as hemophiliacs.
Fibrin from different animal sources is generally glycosylated with complex type biantennary asparagine linked glycans. Variety is just found in the degree of core fucosylation and in the type of sialic acid and galactose linkage.
The image at the left is a crystal structure of the double-d fragment from human fibrin with two bound ligands. The experimental method used to obtain the image was X-ray diffraction, and it has a resolution of 2.30 Å. The structure is mainly made up of single alpha helices shown in red and beta sheets shown in yellow. The two blue structures are the bound ligands. The chemical structures of the ligands are Ca+2 ion, alpha-D-mannose (C6H12O6), and D-glucosamine (C8H15NO6).
See also 
- TGW1916.net, Defibrinated blood harvested from sheep (video)
- Fibrin: Molecule of the Month, by David Goodsell, RCSB Protein Data Bank