Blood clot. Coloured scanning electron micrograph (SEM) of a freeze fracture through a blood vessel showing a blood clot (thrombus). The red blood cells (erythrocytes, red) are trapped in filaments of fibrin protein (grey). Blood clotting is the solidification of blood that occurs when blood vessels are damaged. This damage activates blood cells called platelets, which in turn help to stimulate the formation of fibrin filaments at the site of injury. These filaments enmesh platelets, and red and white blood cells, contracting around them to form a solid clot. Inappropriate clotting is a major cause of strokes and heart attacks.