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.
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.