White blood cells circulate throughout the body and tissues, providing protection against foreign organisms and matter. White blood cells are called leukocytes.

To carry out their intended functions, white cells must be highly mobile. They must be able to squeeze through the pores in the capillaries and move into the tissues. When a foreign organism (referred to as an antigen) enters the body, chemical substances are released that stimulate the white cells and cause them to be attracted to the area of invasion.

The average adult make has 75 billion circulating white blood cells. When an infection occurs in the body, the white cell level of the blood increases. The extent of the increase depends on the length of time the infection has been present. An increased white cell count is a classic sign of infection somewhere in the body.

White blood cells are divided into three groups: the granulocytes, monocytes and lymphocytes. Together, these cells help the body fight infection, play a vital role in the body's immune response, and can help trigger the body's rejection of a transplanted organ. When defective, they can also cause autoimmune diseases such as rheumatoid arthritis, diabetes mellitus and myasthenia gravis.