A low white blood cell count (leukopenia) is a decrease in disease-fighting cells (leukocytes) in your blood. Leukopenia is almost always related to a decrease in a certain type of white blood cell (neutrophil). The definition of low white blood cell count varies from one medical practice to another. In general, for adults a count lower than 4,000 white blood cells per microliter of blood is considered a low white blood cell count. For children, that threshold varies with age. Some people who are otherwise healthy have white cell counts that are lower than what's usually considered normal, but which are normal for them.

Causes of Low White Blood Cells Count
White blood cells are manufactured in bone marrow - the spongy tissue inside some of your larger bones. A low white blood cell count usually is caused by:
-- Viral infections that temporarily disrupt the work of bone marrow
-- Certain disorders present at birth (congenital) that involve diminished bone marrow function
-- Cancer (or other diseases that damage bone marrow)
-- Autoimmune disorders that destroy white blood cells or bone marrow cells
-- Severe infections that use up white blood cells faster than they can be produced
-- Medications, such as antibiotics, that destroy white blood cells
-- Myelodysplastic Syndrome
-- Aplastic anemia
-- Chemotherapy
-- Radiation therapy
-- HIV/AIDS

When to see a doctor
A low white blood cell count is usually found when your doctor orders tests for a condition you're already experiencing. It's rarely an unexpected finding or simply discovered by chance. Talk to your doctor about what your test results mean. A low white blood cell count, along with results from other tests, might already indicate the cause of your illness. Or your doctor may suggest other tests to further check your condition. Because a chronic very low white blood cell count makes you vulnerable to infections, ask your doctor about precautions to avoid catching contagious diseases. Always wash your hands regularly and thoroughly. You might also be advised to wear a face mask and avoid anyone with a cold or other illness.