White blood cells and platelets, coloured scanning electron micrograph (SEM). White blood cells (leucocytes, blue, rounded) are part of the body's immune system. Platelets (thrombocytes, blue-green) are activated when a blood vessel is damaged. The platelets secrete chemicals that cause the formation of a fibrin mesh. This mesh traps platelets and red and white blood cells, forming a clot that seals the damaged blood vessel.