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What Are Eosinophils?

eosinophil surrounded by red blood cells
An eosinophil surrounded by red blood cells.

Credit: NIAID

Eosinophils are specialized white blood cells that participate in inflammatory processes such as allergic diseases. They also are involved in tissue repair and remodeling.

Eosinophils belong to a category of white blood cells called granulocytes. Eosinophils contain small particles, called secondary granules, packed with a variety of proteins and chemicals that regulate immune responses. Activation of eosinophils triggers their release of these chemicals and proteins, a process known as degranulation. In addition to harming intruding pathogens, the substances released during degranulation also can damage the body’s cells and tissues.

In healthy people, eosinophils comprise approximately 1 to 6 percent of white blood cells. The body may produce more of these cells in response to parasitic and fungal infections. Certain allergic diseases, skin conditions, autoimmune disorders, cancers, and bone marrow diseases also may result in elevated eosinophil counts. Many people with eosinophilic disorders have high numbers of eosinophils in their blood or tissues over a long period of time. Sometimes, the presence of excess eosinophils in tissue, called “eosinophilic inflammation,” can result in tissue damage.​​

Last Updated April 23, 2014

Last Reviewed April 23, 2014