Any of several types of blood cells that help defend the body from infection. The different mature forms—granulocytes, including neutrophils (heterophils), basophils, and eosinophils; monocytes, including macrophages; and lymphocytes—have different functions, including ingesting bacteria, protozoans, or infected or dead body cells; producing antibodies; and regulating the action of other leukocytes. They act mostly in the tissues and are in the bloodstream only for transport. Blood normally contains 5,000–10,000 leukocytes per cu mm.

Learn more about leukocyte with a free trial on Britannica.com.