If the skin is breached, the blood clots rapidly to seal the wound, prevent loss of blood and prevent pathogens from entering the body.When platelets come into contact with collagen in the skin or the wall of damaged blood vessels, they are activated. Activated platelets change shape and stick to the walls of the damaged blood vessels.Activated platelets and damaged cells release chemicals such as ADP and other inflammatory mediators that attract more platelets and white blood cells such as macrophages to the site.The platelets secrete granules that contain substances that recruit additional platelets to form a platelet plug sealing off the injured area.The granules also contain serotonin which cause vasoconstriction, reducing the supply of blood to the area.The platelets secrete thromboplastin. This triggers the clotting cascade which results in prothrombin being converted to thrombin. Thrombin adheres with circulating fibrinogen and converts it into insoluble fibrin threads, forming a fibrin network. This fibrin network strengthens the initial platelet plug and traps more platelets and blood cells in the mesh.The clot dries into a hard scab covering the wound. Damaged blood vessels regrow and epidermal cells grow below the scab sealing the wound permanently.