White blood cells (WBCs) are a major component of the body's immune system. A doctor may order a WBC count to assess, diagnose, or rule out infectious and inflammatory diseases, leukemia, lymphoma, and bone-marrow disorders.
A WBC count test measures two things: the total number of WBCs (leukocytes) and the differential count.
The differential count measures the percentages of each type of leukocyte present. White blood cells are composed of granulocytes (neutrophils, eosinophils, and basophils) and non-granulocytes (lymphocytes and monocytes).
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