Coloured scanning electron micrograph (SEM) of two white blood cells (leucocytes, white/blue), red blood cells (erythrocytes, red) and platelets (thrombocytes, green) on a fibrin mesh (brown). Erythrocytes transport oxygen from the lungs to the rest of the body and remove carbon dioxide from the body to the lungs. Leukocytes protect against illness and disease. When an area of the body comes under attack from bacteria, a virus or other foreign invaders, leukocytes will travel to the affected area to destroy the potential threat and prevent illness. When a blood vessel becomes damaged, it sends a signal to thrombocytes who respond by travelling to the damaged area and transforming into their activated form (seen here). They change shape and form long tentacle-like protrusions. Fibrin is a protein formed during the clotting of blood. The thrombocytes clump together and, together with the polymerized fibrin, form a clot over the wound. Magnification: x4500 when printed at 10cm wide