Clinical Laboratory - Hematology

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WBC Differential Count

Types of Leukocytes that make up the WBC Differential

The greatest number of leukocytes are the granulocytes, and most of these are neutrophils, with much smaller numbers of eosinophils and basophils. These cells may only remain in the peripheral blood for a few hours before entering into the tissues. Granulocytes cannot re-enter the peripheral blood, and will die in the tissue site. In the tissues, neutrophils are the mainstay against acute infection, particularly bacterial infections.

An initial neutrophilic response can include increased numbers of less mature neutrophils, called "band" neutrophils being releaased from the marrow into the peripheral blood. This is called a "left shift" because there are more precursors in a maturation sequence diagrammed from left to right. The "band count" is based upon a manual differential, and has some observer variability.

Lymphocytes comprise one of the main components of the mononuclear phagocyte system (lymph nodes, spleen, thymus, bone marrow, liver, tonsils, and an extensive network of lymphatic tissue associated with mucosal tissues in organs such as GI tract, salivary glands, and lungs). Lymphocytes act in recognition of infection, and in mediation of cellular and humoral immune function. Lymphocytes may remain in the peripheral blood for weeks to months, while memory cells can persist for years, and may enter into tissue sites and later re-enter into the blood stream.

Monocytes remain in the peripheral blood for a short period before entering tissue sites for final, specialized differentiation as macrophages. Macrophages can remain in tissues for relatively long periods (months).

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