context: White blood cells, also called leukocytes, fight infection. They move throughout your body in your blood, looking for invaders. And your body is continuously making a fresh supply. Your doctor measures how many of these cells you have by sending some of your blood to a lab to do a complete blood count, or CBC. Your white blood cell count is one of the numbers you get back from this test. It may point toward or confirm a diagnosis, or show whether a treatment is working or not. Most often, a low white blood cell count is nothing to worry about. How many white blood cells ( WBCs) someone has varies, but the normal range is usually between 4,000 and 11,000 per microliter of blood. A blood test that shows a WBC count of less than 4,000 per microliter (some labs say less than 4,500) could mean your body may not be able to fight infection the way it should. A low number is sometimes called leukopenia. Your doctor will do a physical exam and consider symptoms that you have along with your past medical issues to figure out what's behind your result. Bone marrow problems: The spongy center of your bones, which is called the bone marrow, makes blood cells. Low WBC counts are often linked to bone marrow problems. Being around certain chemicals, like benzene and pesticides, as well as some types of cancer and cancer treatments including chemotherapy and radiation, can hurt your bone marrow's ability to make WBCs. Autoimmune disorders: Some autoimmune diseases, like lupus and rheumatoid arthritis, will tell your body to attack and destroy its own WBCs. Infection: Viruses can affect your bone marrow and cause low WBCs for a while. Severe infections, like blood infections, can lead to your body using up WBCs faster than it can make them. HIV kills a specific kind of white blood cell. Medicines: Some drugs, including antibiotics, can destroy WBCs. Nutrition: Not eating well or low levels of certain vitamins, such as folic acid and B12, can affect how your body makes WBCs. Alcohol abuse can mess with the nutrients in your body and with WBC counts, too. Spleen problems: The spleen also makes WBCs. Infections, blood clots, and other problems can make it swell and not work the way it should. This will drop your WBC count. If there's no clear reason for a low white blood cell count, your doctor will probably want to do the test again, or do a differential or "diff" along with the CBC. This other test gives a lot more detail. There are normal ranges for each of the five kinds of WBCs, and some problems only affect one type. The results of a diff could help your doctor narrow down what's going on. Many times, a repeated test will show that your WBC count is normal. Your doctor may want to do more tests, based on the symptoms you have. For instance, you might get checked for strep throat or mono. Other blood tests can look for a viral infection, inflammation, or allergies. The doctor may want to take a sample of your bone marrow to see if it's healthy. When your WBC count is very low, you may need to take steps to avoid an infection. Your doctor may ask you to see a hematologist. This is a specialist who has extra training for diagnosing and treating blood count problems. If your WBC count stays low or keeps getting lower, work with your doctor to find out why it's happening. The right treatment should help your white blood cell count return to normal.