Definition of Eosinophil

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Eosinophil: A type of leukocyte (white blood cell) with coarse round granules of uniform size within its cytoplasm and typically a bilobate (two-lobed) nucleus. Eosinophils are so named because their cytoplasmic granules stain red with the dye eosin. Eosinophils normally constitute 1 to 3% of the peripheral blood leukocytes, at a count of 350 to 650 per cubic millimeter. Also called an eosinophilic leukocyte.

The numbers of eosinophils in blood often rise above the normal range with allergic reactions and parasitic infections as with worms. See: Eosinophilia.