Eosinophils are white blood cells that are responsible for combating infection by parasites in the body.

These cells, transparent in vivo, appear brick-red when stained by the Romanowsky method. The red color is visible as small granules within the cell. These granules contain histamine and other chemicals that are toxic to both parasites and the host's tissues. The Eosinophils make up about 1.5% of the total White Blood cells.

Eosinophils also play a role in the allergic response, and in fibrin removal in inflammation. Eosinophils are considered the main effector cells in asthma pathogenesis and are associated with disease severity.

Eosinophils also have a role in fighting viral infections which is evident from the abundance of RNAses they contain within their granules.

An increase in eosinophils is called an eosinophilia, and is typically seen in people with a parasitic infection of the intestines. In 1989, contaminated l-tryptophan supplements caused a deadly form of eosinophilia known as eosinophilia-myalgia syndrome.