Leukopenia is the medical term for a low white blood cell count, according to Mayo Clinic. Doctors generally consider a patient to have leukopenia if he has fewer than 4,000 white blood cells per microliter of blood, although this number can vary depending on age and sex of the patient.
White blood cells, or leukocytes, are part of the body's immune system. They develop in the bone marrow and circulate through blood and lymph tissue, according to the National Cancer Institute. These cells are necessary to identify and fight disease-causing microbes as well as inflammation, allergies and leukemia. Not only does a low white blood cell count impair the body's ability to fight infection, but many pathogens that cause leukopenia may also cause infectious disease, such as the AIDS-causing HIV virus, reports Mayo Clinic. Some prescription and illegal drugs can also disrupt bone marrow function and decrease white blood cell counts in the blood.
Many autoimmune and congenital disorders can also cause leukopenia, notes Mayo Clinic. Some cancers, such as leukemia, and cancer treatments, such as chemotherapy, damage bone marrow, where white blood cells are produced. Aplastic anemia, lupus, and Kostmann's syndrome also damage neutrophil and leukocyte production when left untreated.