White blood cells, also called leukocytes, help the body fight infection. Leukopenia is a medical condition in which there are not enough white blood cells circulating in the blood. Many diseases can cause leukopenia due to damage of the bone marrow, but low white blood cell counts can also be caused by drugs and laboratory errors.
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Treatment for cancer often involves destroying cancer cells with radiation therapy or chemotherapy. While these treatments effectively destroy malignant cells, they also destroy healthy cells that have important functions in the body. White blood cells may be destroyed during a course of radiation therapy or chemotherapy, and the count typically remains low until treatment has been completed.
Liver or Spleen Disorders
Disorders of the liver and spleen may cause cells to be destroyed before they have carried out their functions. In hypersplenism, the spleen destroys blood cells and causes a low white cell count. When these cells are destroyed prematurely, they are unable to fight infection and help the body stay healthy.
Some drugs are known to destroy white blood cells or damage the bone marrow so that white blood cell production is reduced. Prednisone, which is often used for asthma, psoriasis, arthritis and other disorders, is a corticosteroid that is known to cause a decreased white blood cell count. Some antibiotics and diuretics can also cause a drop.
Bone Marrow Failure
Because white blood cells are produced in the bone marrow, bone marrow failure can cause low counts; it may fail because of medical conditions or drugs that damage the marrow. Aplastic anemia is a blood disorder that causes the bone marrow to stop producing new blood cells altogether, and myelodysplastic syndrome causes the bone marrow to produce too few. These and other diseases can cause low white blood cell counts.
Autoimmune diseases, also known as collagen-vascular diseases, cause the immune system to attack healthy cells because of an inability to differentiate between healthy cells and disease-causing organisms. When healthy cells are attacked, a low white cell count may occur. Autoimmune diseases include lupus, rheumatoid arthritis and systemic sclerosis.
Some viruses can cause reduced white blood cell counts because they disrupt the normal function of the bone marrow. When bone marrow function is disrupted, white blood cell production may be reduced. The human immunodeficiency virus (HIV) is an example of a viral infection that can cause a low white blood cell count.
Laboratory errors may be responsible for low white cell counts. Because blood specimens are handled by several people and processed by machines, there is room for error. The blood sample may be too small to get an accurate white blood cell count. If the sample is processed incorrectly, the reported white blood cell count may be incorrect. If a laboratory error is suspected, a new sample must be taken to determine your count.