Molecular model of a fibrin monomers forming a network. Each fibrin monomer is composed of 2 alpha, 2 beta, 2 gamma and 2 alphaC domains. Cleaved ends of the alpha domain (light blue) bind to gamma domains of adjacent monomers and begin to form a network of fibrin molecules. A fibrin mesh is formed in response to chemicals secreted by platelets, fragments of white blood cells. Clots are formed in response to cardiovascular disease or injuries to blood vessels.