Low white blood cell counts may occur when patients have conditions that interfere with bone marrow functioning or conditions like cancer, viral infections and autoimmune disorders, according to Mayo Clinic. Severe infections may also cause the body to use more white blood cells than it can produce.
White blood cells, also called leukocytes, help the body fight infections, notes MedlinePlus. Physicians order white blood cell counts as part of a complete blood count during routine exams, according to the American Association for Clinical Chemistry. The test may also be performed if an patient displays symptoms of an infection, a blood disorder or an impaired immune system.