Blood clot. Coloured scanning electron micrograph (SEM) of a blood clot on the surface of the skin. The red blood cells (erythrocytes) are trapped in filaments of fibrin protein (yellow strings). Blood clotting is the solidification of blood that occurs when blood vessels are damaged. This damage activates blood cells called platelets (not seen), which in turn help to stimulate the formation of fibrin filaments at the site of injury. These filaments enmesh platelets, and red & white blood cells, contracting around them to form a solid clot. Inappropriate clotting is a major cause of strokes and heart attacks.