White blood cells, also called leukocytes, help the body fight infection. Five types of white blood cells -- each with a unique disease-fighting purpose -- exist in varying amounts in the blood. The five types are basophils, eosinophils, lymphocytes, monocytes and neutrophils.
A blood differential is a diagnostic test that counts and compares the amounts of all five types of leukocyte present in the blood. A lower than normal white blood cell count is called leukopenia. It can be indicative of a number of different health conditions.
Viral Infections
Viral infections can cause leukopenia by attacking the bone marrow where white blood cells are formed. In the case of HIV/AIDS, the virus attaches to, and enters, certain lymphocytes and replicates itself inside them. The newly made copies of the replicated virus then invade the bloodstream and attack other cells. The original host cell dies, and the process repeats until lymphocyte levels are severely depleted and the body is unable to fight off infections, according to the Mayo Clinic.
Cancers That Destroy Bone Marrow
Leukemia is a cancer that attacks the bone marrow and lymphatic systems, both of which are vital in the formation of white blood cells. Leukemia causes the bone marrow to produce a proliferation of abnormal, non-functioning white blood cells.
Autoimmune Disorders
Lupus erythematosus is an autoimmune disorder, meaning that it causes the immune system to attack healthy tissue. This results in chronic inflammation. The Mayo Clinic reports that the causes of lupus are not known, but blood tests of lupus patients often reveal low white blood cell counts.
Infections That Use Up White Blood Cells
Severe infections, such as sepsis, may overwhelm the body's ability to create enough leukocytes to combat the disease. Sepsis is a bacterial infection of the bloodstream. It can be fatal. White blood cell counts can be abnormally low, or high, in cases of sepsis, and a blood differential may reveal the presence of white blood cells that have failed to mature.
Drugs That Destroy Bone Marrow
Certain chemotherapy drugs that are used to treat cancer can kill bone marrow cells, thus reducing the body's capacity for making white blood cells, according to the Mayo Clinic. Because of this, the doctors who administer these types of chemotherapy commonly monitor blood cell counts. If the count falls too low, the doctor may use blood transfusions, medications or temporary cessation of treatment until the blood cell count rises to normal levels again.
