White blood cell: One of the cells the body makes to help fight infections. There are several types of white blood cells (leukocytes). The two most common types are the lymphocytes and neutrophils (also called polymorphonuclear leukocytes, PMNs, or "polys").
Lymphocytes are made in lymphoid tissue in the spleen, lymph nodes, and thymus gland. There are different kinds of lymphocytes. Lymphocytes identify foreign substances from germs (bacteria or viruses) in the body and produce antibodies and cells that specifically target them. It takes from several days to weeks for lymphocytes to recognize and attack a new foreign substance.
Neutrophils are also major players in the body's defense against bacterial infections. Neutrophils are made in the bone marrow and circulate in the bloodstream. Neutrophils move out of the blood vessels into the infected tissue to attack the bacteria. The pus in a boil (an abscess) is made up largely of neutrophils. Normally a serious bacterial infection causes the body to produce an increased number of neutrophils, resulting in a higher than normal white blood cell count (WBC). When the WBC is low, there may not be enough neutrophils to defend against bacterial infections.
The white blood cell count is done by counting the number of white blood cells in a sample of blood. A normal WBC is in the range of 4,000 to 11,000 cells per microliter. A low WBC is called leukopenia. A high WBC is termed leukocytosis.
A normal absolute neutrophil count (ANC) is in the range of 1,500 to 8,000 cells per microliter. If the ANC is below 500 for an extended period of time, the risk of serious bacterial infection may increase significantly. A low neutrophil count is called neutropenia.