Leukocyte

Definition
noun, plural: leukocytes
Any of the nucleated blood cells that lack hemoglobin, and whose main function involves the body’s immune system, protecting the body against invading microorganisms and foreign particles
Supplement
Blood is the circulating fluid in the body of eukaryotic animals. It is primarily composed of plasma and cellular elements (blood cells and platelets). The blood cells with nuclei and lacking in hemoglobin are the white blood cells, as opposed to other blood cell types such as red blood cells. 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. Similar to red blood cells, the white blood cells (also called leukocytes) originate from a hematopoietic stem cell (hemocytoblast). The hemocytoblast gives rise to progenitor cells of both the myeloid series and the lymphoid series. Leukocytes from the myeloid series include the granulocytes and the monocytes whereas those from the lymphoid series include the lymphocytes and the lymphoid dendritic cells.
Different methods of classifying leukocytes:

• Based on the presence of granules in the cytoplasm (present or absent, or more precisely, visible on light microscopy or not thus visible): granulocytes (with distinctive cytoplasmic granules, e.g. neutrophils, basophils, eosinophils, and mast cells) vs. agranulocytes (apparently lacking granules, e.g. lymphocytes, monocytes and macrophages)
• Based on the nuclear lobes: polymorphonuclear (i.e. a nucleus with more than one lobe) leukocytes vs. mononuclear (i.e. a nucleus with only one lobe) leukocytes

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: Greek leukos – white, and kytos – cell.

Related forms: leukocytic (adjective)
Variant(s):

• leucocyte (British)

Synonym(s):

white blood cell (or WBC)

White blood corpuscle

white corpuscle

white cell

See also:

• blood
• immune system
• basophil
• eosinophil
• lymphocyte
• neutrophil
• monocyte
• macrophage
• dendritic cell

Related term(s):

Agranular leukocyte

Automated differential leukocyte counter

Basophilic leukocyte

Eosinophilic leukocyte

Granular leukocyte

Human leukocyte antigen

Human leukocyte-associated antigens

Leukocyte inclusion

Leukocyte interferon

Leukocyte migration-inhibitory factors

Leukocyte transfusion

Leukocyte-adhesion deficiency syndrome

Mast leukocyte

Motile leukocyte

Polymorphonuclear leukocyte

Receptors leukocyte-adhesion

Turks leukocyte

Related form(s):

• leukocytic (adjective, of, pertaining to, relating to, or characterized by a leukocyte)