Animation showing the formation of a blood clot, or thrombus. Clotting is initiated by platelets (white), small blood cells that are activated when they encounter a damaged portion of a blood vessel wall. The platelets develop long protrusions, and stick to the wall and each other. They also interact with proteins in the blood to form long strands of the protein fibrin (grey). The mesh of fibrin and activated platelets traps red blood cells, and forms a plug that seals the damaged vessel, preventing blood loss and exposure to pathogens.