White blood cells (also called leukocytes or immune cells) are cells which form a component of the blood.
They help to defend the body against infectious disease and foreign materials as part of the immune system.
There are normally between 4x109 and 11x109 white blood cells in a litre of healthy adult blood - about 7,000 to 25,000 white blood cells per drop.
In conditions such as leukaemia this may rise to as many as 50,000 white blood cells in a single drop of blood.
As well as in the blood, white cells are also found in large numbers in the lymphatic system, the spleen, and in other body tissues.