White blood cells play an important role in the immunity/immune system of an individual. The white blood cell count is done routinely as part of a complete blood count. An unusually high white blood cell count can indicate an infection, hypersplenism, bone marrow depression (drugs, radiation or heavy metal poisoning) or primary bone marrow disorders such as leukemia. A low white blood cell count can be the result of infection, make an individual more susceptible to outside infections or allow multiplication of organisms within the body which would normally kept in check by a healthy immune system. There are many different types and forms of white cells. Since deficiencies can affect one or more types or forms, a doctor's help and additional testing is usually required to understand the nature or cause of the deficiency.

Neutropenia, the most common cause of a low white count, can occur in acute bacterial infections, viral infections, rickettsiae disease, alcoholism, some parasite injections, drug use, aplastic and pernicious anemia, acute lymphoblastic leukemia, some hormone imbalances and anaphylactic shock.

The therapeutic use of a granulocyte colony-stimulating factor (expensive) can increase the number of circulating neutrophils, enhance neutrophil bactericidal activity, and can be of benefit in reducing morbidity and mortality from infectious disease in those with very low levels of neutrophils.