White blood cells play an important role in the immune system of an individual. The white blood cell count is done routinely as part of a complete blood count and further studies are usually needed if abnormalities are discovered through this test. An unusually high white blood cell count can indicate an infection or a blood disorder such as leukemia. However, a low white blood cell count will make an individual susceptible to infectious diseases or allow multiplication of organisms within the body which are normally kept in check by the immune system.

Further evaluation of the types of white blood cells which are increased will help to determine the cause.