What is an Eosinophil?

Eosinophils, a type of white blood cell, are an important part of the immune system, helping us fight off certain types of infections, such as parasites. They are named because of the characteristic microscopic stain that gives them a reddish color under the microscope. Many different problems can cause high numbers of eosinophils in the blood including allergies (food and environmental), certain infections (caused by parasites), eosinophil associated gastrointestinal disorders, leukemia, and other problems. When eosinophils occur in higher than normal numbers in the body, without a known cause, an eosinophilic disorder may be present. Sometimes, eosinophils can be found in tissues in addition to the blood circulation which is seen in certain types of disease states. Typically, eosinophils make up less than 5% of circulating white blood cells in health individuals and can vary over time in healthy individuals.

Eosinophilic disorders are further defined by the area of the body affected. For instance, eosinophilic esophagitis is associated with abnormally high numbers of eosinophils in the esophagus.

Eosinophil, Courtesy of Dr. Margaret Collins

© American Partnership for Eosinophilic Disorders 2008, 2010, 2011. Content may not be reproduced in part or in whole without express written consent from APFED. Author Wendy Book MD, Margaret Collins MD
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