Multiple causes exist for low white blood cells. In almost all cases, while depending on the specific type of white blood cells, there are recognizable symptoms. Most of the symptoms cluster around typical results of infectious processes or diseases. However, a detailed blood analysis is necessary to identify the exact nature of the low level of these cells.
The bone marrow produces white blood cells (leukocytes). Normally about 100 billion leukocytes are created in the body each day. Blood tests measure the level of leukocytes in terms of number of leukocytes per microliter of blood. A normal value usually runs between 4,000 and 11,000 cells per microliter. If something causes the level to fall below 4,000, it is a sign of a disease process and makes a person more open to infections.
There are five types of leukocytes: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Neutrophils constitute the body's defense against invading bacteria and fungus infections. About 45 to 75 percent of leukocytes in the blood are neutrophils. A low value is anything below 1,000 per microliter. Lymphocytes represent between 20 to 40 percent of white blood cells. When the level drops not too far below 1,500 cells per microliter, there may be no symptoms, but lower levels open the opportunity for infection. Monocytes are a small fraction of leukocytes---one to 10 percent. A healthy level is between 200 to 600 cells per microliter. Eosinophils represent about 7 percent of circulating leukocytes. A low level may not cause particular symptoms. Basophils can number anywhere from zero to 300 per microliter.
Over all there are at least 31 different causes for low white blood cells. These include leukemia, aplastic anemia, chemotherapy, radiation treatment, lupus, diseases of the liver and spleen, and various childhood and adult disease such as measles, flue, malaria and typhoid. Certain drugs such as doxorubicin hydrocholoride, ceflizoxime and chlorpromazine as well as drug interactions may lead to leukopenia, Low lymphocytes may result from human immunodeficiency virus (HIV). Treatment with corticosteroids, chemotherapy or radiation therapy for cancer also can cause this condition. It can even happen just during acute stress. Low monocyte levels are caused when some bacteria release toxins or from chemotherapy or corticosteroids. Low levels of eosinophils may happen with Cushing's syndrome, a disorder of the adrenal glands or stress.
The numbers of white blood cells in the body are determined through a blood test called a complete blood count, commonly abbreviated as CBC. When it is necessary or desirable to know the breakdown of each type, this is called a differential blood count. The count can be done manually or by an automated counter. Manual counts involve putting a smear of blood on a slide and then counting and identifying a specific number of cells. Automated counts, while faster, are felt not to be as accurate.
Low values of white blood cells have different symptoms and causes. A low number of neutrophils is called neutropenia. Generally when the bone marrow cannot produce enough to make up for the ones being destroyed through bacterial or viral infections, allergies, some drugs, cancer or radiotherapy for cancer, neutropenia will develop. When neutrophils fall below the expected level, the person will be even more susceptible to infectious diseases. Since monocytes become macrophages, which are the primary scavenger cells in the body's immune system, low levels can affect immunity. Eosinophils also play a role in the immune system so that deceases can have similar results. Basophils affect immunity and repair of wounds, which are affected by low levels of these cells.