Eosinophils are white blood cells that are responsible for combatting infection by parasites that enter the bloodstream.

These cells, transparent in vivo, appear brick-red when stained by the Romanowsky method. The red colour 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.

Eosinophils also play a role in the allergic response, and in fibrin removal in inflammation.

An increase in eosinophils is called an eosinophilia, and is typically seen in people with a parasitic infection of the intestines.