- Hypereosinophilic syndrome (HES) is a disorder in which the body has too many eosinophils (a type of white blood cell). Too many eosinophils in HES can cause damage to the heart, nerves, or skin. Certain drugs can help lower eosinophil counts to prevent tissue damage. Corticosteroids, such as prednisone, are used for initial therapy in this disorder. Although most people respond to prednisone, some people develop side effects from it, or do not respond very well to treatment. Better ways of determining the dose to give could help to decide on the best therapy for HES.