Eosinophil: A normal type of white blood cell that has coarse granules within its cytoplasm. Eosinophils are produced in the bone marrow and migrate to tissues throughout the body. When a foreign substance enters the body, other types of white blood cells (lymphcytes and neutrophils) release substances to attract eosinophils and then release toxic substances to kill the invader. The numbers of eosinophils in blood often rise when an allergic reaction occurs. Elevated eosinophil counts are also common in some diseases, such as parasite diseases and asthma.