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Eosinophil granulocytes, or simply eosinophils, from the Greek for "acid loving", are a type of white blood cell. They are created in the bone marrow. They play a key role in the immune system's response to parasites and some infections. On the other side, they also play a role in allergies and asthma.

Normally, anywhere between 1-6% of all white blood cells are eosinophils. Although they are found in several parts of the body, they normally do not appear in the lungs, skin or esophagus.

The presence of an abnormally large number of eosinophils (more than 500 in 1 microlitre of blood) is usually associated with asthma, but can indicate any of a number of conditions from arthritis to Hodgkin's disease and Addison's disease.

Many drugs can suppress the action of eosinophils, such as steroids.

Eosinophil granulocyte at Wikipedia