Eosinophil is a type of white blood cell that fights parasites and bacteria. Eosinophils belong to the group of leukocytes (white blood cells) known as granulocytes, which means that they are characterized by the presence of large red granules in their cytoplasms. When foreign microorganisms enter the body, lymphocytes and neutrophils (types of leukocytes) release certain substances that attract eosinophils which release toxic substances to kill the invaders.
Eosinophils are transparent, but they show brick-red under the microscope after staining with eosin, a dye, using the Romanowsky method. They measure between 12 and 17 micrometers in size. Only 6% of white blood cells are eosinophils. Aside from destroying bacteria, eosinophils attack parasitic larvae and their number increase in the presence of certain parasites. Eosinophils along with basophils are also important mediators of allergic responses and asthma pathogenesis. Eosinophils are produced in the bone marrow.