Single white blood cell (leucocyte or leukocyte), coloured scanning electron micrograph (SEM). 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 and prevent illness. Magnification: x9500 when printed at 10cm wide