Blood clot. Coloured scanning electron micrograph (SEM) of blood clotting in an ovarian follicle. The red blood cells (erythrocytes) are trapped in filaments of fibrin protein (brown). 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 and white blood cells, contracting around them to form a solid clot. The blood has been released from blood vessels damaged when the follicle collapsed after ovulation (release of an egg).