Cleaved by the protease thrombin to yield monomers which, together with fibrinogen beta (FGB) and fibrinogen gamma (FGG), polymerize to form an insoluble fibrin matrix. Fibrin has a major function in hemostasis as one of the primary components of blood clots (PubMed:7649481). In addition, functions during the early stages of wound repair to stabilize the lesion and guide cell migration during re-epithelialization (PubMed:11389004). Was originally thought to be essential for platelet aggregation, based on in vitro studies using anticoagulated blood (PubMed:7649481). However, subsequent studies have shown that it is not absolutely required for thrombus formation in vivo (PubMed:10930441). Enhances expression of SELP in activated platelets via an ITGB3-dependent pathway (PubMed:19332769). Maternal fibrinogen is essential for successful pregnancy (PubMed:7649481). Fibrin deposition is also associated with infection, where it protects against IFNG-mediated hemorrhage (PubMed:12629066). May also facilitate the immune response via both innate and T-cell mediated pathways (PubMed:23487423). Heterohexamer; disulfide linked. Contains 2 sets of 3 non-identical chains (alpha, beta and gamma). The 2 heterotrimers are in head to head conformation with the N-termini in a small central domain. Expressed in liver. A long coiled coil structure formed by 3 polypeptide chains connects the central nodule to the C-terminal domains (distal nodules). The long C-terminal ends of the alpha chains fold back, contributing a fourth strand to the coiled coil structure. Conversion of fibrinogen to fibrin is triggered by thrombin, which cleaves fibrinopeptides A and B from alpha and beta chains, and thus exposes the N-terminal polymerization sites responsible for the formation of the soft clot. The soft clot is converted into the hard clot by factor XIIIA which catalyzes the epsilon-(gamma-glutamyl)lysine cross-linking between gamma chains (stronger) and between alpha chains (weaker) of different monomers. Forms F13A-mediated cross-links between a glutamine and the epsilon-amino group of a lysine residue, forming fibronectin-fibrinogen heteropolymers. Phosphorylated by FAM20C in the extracellular medium. Knockout mice are viable but only males are fertile (PubMed:7649481). Neonates frequently develop spontaneous hemorrhages, but in spite of this over 90% of mice survive the neonatal period (PubMed:7649481). However only half survive beyond 70 days of age; lethality is most often due to intra-abdominal hemorrhage (PubMed:7649481). Pregnancy in female mice fails at around 10 days of gestation, associated with severe intrauterine bleeding (PubMed:7649481). Secondary loss of FGB and FGG from circulating blood is observed, although FGB and FGG mRNA is normally expressed in hepatocytes (thought to be the main site of fibrinogen synthesis) (PubMed:7649481). In vitro, blood fails to clot and platelet aggregations do not form (PubMed:7649481). In vivo, platelet aggregation in injured arterioles initially occurs normally although thrombi are unstable and readily embolize (PubMed:10930441). In double knockouts of FGA and VWF, platelet aggregation is delayed and thrombi frequently embolize (PubMed:10930441). Mice succumb more rapidly to Y. pestis infection, associated with increased bacterial loads in liver and lung; however induction of the inflammatory response factors TNF, IFNG, CXCL1, and LCN2 is not affected (PubMed:23487423). Mice succumb more rapidly to T. gondii infection, with increased hemorrhagic pathology; however parasite numbers are not significantly increased and induction of the inflammatory response markers IL12, IFNG, TNF, IL10, and nitric oxide is not affected (PubMed:12629066). Mice succumb more rapidly to L. monocytogenes infection, with increased hemorrhagic pathology and increased bacterial burdens in hepatic tissue; however there is little effect on peritoneal bacterial numbers or bacterial dissemination to other tissues, and also no effect on induction of the inflammatory markers IFNG, TNF and NOS2 (PubMed:15972474).