A leukocyte, or white blood cell, is a biological cell involved in immune defence. The category includes lymphocytes and cells such as macrophages and neutrophils, whose recognition and effector mechanisms differ. Many leukocytes circulate in blood but act within tissues, so a blood count samples only part of their distribution. Unlike the name might suggest, the category describes cellular identity rather than requiring a visibly white individual cell. [ref]
Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others. Chapter 1, The Components of the Immune System
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