Leukocytes, also known as white blood cells, patrol the blood and lymphatic system in various forms, destroying bacteria and other disease-causing, foreign bodies through ingestion, enzyme manufacture, secretion of toxins or antibodies, antigen boost or other assistive immune responses. While leukocytes make up only about 1 percent of adult blood volume, they are essential to immune system health.
In healthy adults, a normal WBC count is in the range of 4,000 to 11,000 white blood cells per microliter of blood. Numbers outside this range can lead to autoimmune disorders, but even normal counts can belie disease if the leukocytes fail to function properly, such as in the case of leukemia (cancer of the WBC). Certain medications, radiation, alcohol abuse, and organ transplant or other major surgeries can all suppress WBC counts, increasing the risk for infection. Conversely, strenuous exercise, rheumatoid arthritis, stress, steroid use, and cigarette smoking (up to five years after quitting) raise WBC counts, but only require medical intervention if autoimmune complications develop. It is also normal to see elevated counts in pregnant women.
The term "leukocyte" is derived from Greek, meaning "white (leuko), hollow vessel (kytos), cell (cyte)," so named because they are typically found in the thin, white layer of cells between the red blood cells and the plasma in centrifuged samples.