Eosinophils are a type of white blood cell. They contain particles filled with chemicals that fight off infections and play a role in your body's immune response. Normally your blood doesn't have a large number of eosinophils. Your body may produce more of them in response to allergic disorders, inflammation of the skin, and parasitic infections. They can also increase in response to some infections or to some bone marrow disorders. In some conditions, the eosinophils can move outside the bloodstream and into organs and tissues. Treatment of the problem depends on the cause.