A congenital or autoimmune disorder, cancer, a viral infection, an overwhelmingly strong infection, or certain drugs may cause a low white blood cell count, Mayo Clinic advises. Damage to bone marrow, which produces white blood cells, or to white blood cells themselves reduces white blood cell levels.
The role of white blood cells is to fight infections, MedicineNet states. Lymphocytes and neutrophils are the two most commonly found kinds of white blood cells. Lymphocytes are created in lymphoid tissue, and they are further subdivided into multiple types. They fight infections by releasing antibodies tailored against specific substances released by viruses and bacteria. This process may take days to weeks, whereas neutrophils act more quickly. Unlike lymphocytes, neutrophils attack bacteria directly, and they are created in bone marrow. An infection site receives a large amount of neutrophils, which arrive via the bloodstream and fight bacteria at the site. In certain cases (boils, for example), this may result in pus, which consists mostly of neutrophils.
A white blood cell count of under 4,500 per microliter of blood is considered to be below the normal range, MedlinePlus states. Such a white blood cell count makes the patient more susceptible to infections. Having fewer than 1,700 neutrophils per microliter is considered low, and a level of fewer than 500 strongly increases the patient’s risk of infection.