The most likely explanation is that the low count is normal for that person. This is especially likely if the count is only slightly low, if there's been no history of unusual or frequent infections, and if no other explanation can be identified.

White blood cells are part of the immune system. It's important to have enough white blood cells. A history of recurrent infections suggests that the low count is either directly responsible; or that the low white cell count is related to an underlying disease that alters the immune system.

Definitions of "low" or "high" white blood cell counts are created by measuring counts in many normal, healthy people. The results that represent the middle 95% of those tested are considered normal. By this method, 5% of the healthy population (those with the highest 2.5% and the lowest 2.5%) will have "abnormal" results. This is a statistical variation that is usually meaningless.

There are other possibilities, of course, because there are many causes of a low white blood cells count. Most of these are found in people who are sick or who have a readily identifiable reason.

The white blood cell count may be low due to:

• Infection, especially viral infections such as mononucleosis or even the common cold
• Toxins (such as alcohol) or medications
• Bone marrow diseases, including certain types of cancer
• Autoimmune diseases, such as lupus, in which the immune system attacks blood cells
• An enlarged spleen, as is often the case in people with cirrhosis
• Congenital (life-long) bone marrow disorders
• Prior radiation therapy
• A vitamin deficiency, such as a low level of folic acid

For most of these, doctors can usually establish a diagnosis readily by reviewing the medical history, performing a physical examination and ordering a limited number of tests.