Red blood cells (erythrocytes) and a single white blood cell (leucocyte or leukocyte) in a fibrin mesh, coloured scanning electron micrograph (SEM). Formation of a blood clot with many erythrocytes (red) and a single leukocyte (white/blue) becoming entangled in a fibrin mesh (light brown). Although only accounting for approximately 1% of blood, leukocytes are essential for good health and work to protect us against illness and disease. They are made in the bone marrow and are stored in the blood and lymphatic system. When an area of the body comes under attack from bacteria, a virus or other foreign invaders that may be a threat to your health, leukocytes will travel to the affected area to destroy the potential threat. Fibrin is a protein formed from fibrinogen during the clotting of blood. It forms a mesh that disrupts the flow of blood thus preventing additional blood loss from the site of a wound. Magnification: x2100 when printed at 10cm wide