Leukopenia is an abnormal decrease in the number of leukocytes (white blood cells). A person having fewer than 4000 total white blood cells per microliter (µL) of blood is at risk for infection.
This blood disorder is often a symptom of some cancers and immune system disorders like AIDS. Chemotherapy drugs used to treat cancers can also often cause leukopenia. Infectious diseases like acute histoplasmosis that can overwhelm the immune system can also cause a drop in white blood cell counts.
If a person's leukopenia is caused by a bacterial infection, the treatment is to give them antibiotics. If it is drug-induced, the treatment is to stop giving the person the toxic medications for a while to let his or her bone marrow recover and start producing more white blood cells.
Chronic leukopenia can also be treated with growth factors such as cytokines and granulocyte-macrophage colony-stimulating factor (GM-CSF). People with severe reductions in their neutrophil counts combined with an enlarged spleen sometimes need to have that organ surgically removed.
Most of the information in this writeup was gleaned from http://www.merck.com/pubs/mmanual/section11/chapter135/135a.htm. The rest is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/