Fibrin is made from fibrinogen, a soluble plasma protein that is produced by the liver. Processes in the coagulation cascade activate the enzyme thrombin, which is responsible for converting fibrinogen into fibrin. Fibrin can then polymerise and form a clot.
The liver produces fibrinogen, which normally has a concentration between 1.8 - 4.0 g/L in blood plasma. In its natural form, fibrinogen is useful in forming bridges between platelets, by binding to their GpIIb/IIIa membrane proteins; though fibrinogen's major use is as a precursor to fibrin.
Fibrinogen is a dimer (it is composed of two identical units). Each of these units has an alpha, a beta, and a gamma chain. On the alpha and beta chains, there is a small peptide sequence (called a fibrinopeptide). It is these small peptides that prevent fibrinogen spontaneously forming polymers with itself.
Processes in the coagulation cascade activate a protease enzyme known as thrombin (or activated Factor II). Thrombin cleaves the fibrinopeptides off the alpha and beta chains of fibrinogen, converting it to fibrin.
The central units of fibrin are now free to react with the terminal units of the other molecules (as the fibrinopeptides blocking this reaction have been removed).
The active molecules of fibrin stack up on each other, usually incorporating (by trapping) aggregrates of platelets. The fibrin molecules are later cross-linked (by factor XIII) covalently, to form a stable haemostatic plug, thus effectively stopping bleeding.