White cells are motile with specialised functions.
The number of leucocytes in normal blood ranges between 4,500 and 11,000 per mm3. Physical exertion may cause the count to exceed 20,000 per mm3. Most of the leucocytes are outside the circulation, and the few in the bloodstream are in transit from one site to another. The most numerous of the leucocytes, are important mediators of the inflammatory response. Neutrophils live only a few hours in the blood, some migrate to the tissues to areas of infection or injury.