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noun, plural: leukocytes

Any of the blood cells that lack hemoglobin, colourless and with nucleus. Its major function involves the body's immune system, protecting the body against invading microorganisms and foreign particles.


They are produced and derived from a multipotent cell in the bone marrow, called hematopoietic stem cell. They are called white blood cells because when whole blood is centrifuged, these cells separate into a thin layer that is typically white in color.

There are two types of leucocytes according to the presence of differently staining granules in their cytoplasm. The granulocytes (also known as polymorphonuclear leucocytes) are leucocytes with very distinctive cytoplasmic granules, e.g. neutrophils, basophils and esoniphils. The second is agranulocytes (mononuclear leucocytes), which are characterized by the lack of apparent granules, e.g. lymphocytes, monocytes and macrophages.

Leukocyte count above the normal range indicates inflammation or infection. In adult humans, the normal range is between 4500 and 11 000 per mm3.

Word origin: from Greek leukos - white, and kytos - cell.
Related forms: leukocytic (adjective)

Variant: leucocyte (British)


Compare: erythrocyte
See also: basophils, coelomocytes, eosinophils, haemocytes, lymphocytes, neutrophils, monocytes.