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[leuko- + -cyte]
Any of several kinds of colorless or nearly colorless cells of the immune system that circulate in the blood and lymph. Leukocytes comprise granulocytes and agranulocytes.
SYN: SEE: white blood cell; SEE: white cell; SEE: white blood corpuscle; SEE: white corpuscle
SEE: blood for illus
Neutrophils, 55% to 70% of all leukocytes, are the most numerous phagocytic cells and are a primary effector cell in inflammation. Eosinophils, 1% to 3% of total leukocytes, destroy parasites and are involved in allergic reactions. Basophils, less than 1% of all leukocytes, contain granules of histamine and heparin and are part of the inflammatory response to injury. Monocytes, 3% to 8% of all leukocytes, become macrophages and phagocytize pathogens and damaged cells, esp. in the tissue fluid. Lymphocytes, 20% to 35% of all leukocytes, have several functions: recognizing foreign antigens, producing antibodies, suppressing the immune response to prevent excess tissue damage, and becoming memory cells.

Leukocytes are formed from the undifferentiated stem cells that give rise to all blood cells. Those in the red bone marrow may become any of the five kinds of leukocytes. Those in the spleen and lymph nodes may become lymphocytes or monocytes. Those in the thymus become lymphocytes called T lymphocytes.

Leukocytes are the primary effector cells against infection and tissue damage. They not only neutralize or destroy organisms, but also act as scavengers, engulfing damaged cells by phagocytosis. Leukocytes travel by ameboid movement and are able to penetrate tissue and then return to the bloodstream. Their movement is directed by chemicals released by injured cells, a process called chemotaxis. After coming in contact with and recognizing an antigen, neutrophils or macrophages phagocytize (engulf) it in a small vacuole that merges with a lysosome, to permit the lysosomal enzymes to digest the phagocytized material. When leukocytes are killed along with the pathogenic organisms they have destroyed, the resulting material is called pus, commonly found at the site of localized infections. Pus that collects because of inadequate blood or lymph drainage is called an abscess.

Microscopic examination: Leukocytes can be measured in any bodily secretion. They are normally present in blood and, in small amounts, in spinal fluid and mucus. The presence of leukocytes in urine, sputum, or fluid drawn from the abdomen is an indication of infection or trauma. The type of white blood cell (WBC) present is identified by the shape of the cell or by the use of stains (Wright) to color the granules: granules in eosinophils stain red, those in basophils stain blue, and those in neutrophils stain purple.

Clinically, WBC counts are important in detecting infection or immune system dysfunction. The normal WBC level is 5,000 to 10,000/mm3. An elevated (greater than 10,000) leukocyte count (leukocytosis) indicates an acute infection or inflammatory disease process (such as certain types of leukemia), whereas a decrease in the number of leukocytes (less than 5,000) indicates either immunodeficiency or an overwhelming infection that has depleted WBC stores. In addition to the total WBC count, the differential count is also frequently important. A differential count measures the percent of each type of WBC, e.g., neutrophils, monocytes, lymphocytes. The differential also measures the number of immature cells of each cell type as an indication of production by the bone marrow. Immature cells are called blasts, e.g., lymphoblasts, myeloblasts. During infections or in certain types of leukemia, blasts may be present in peripheral blood.
SEE: inflammation

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