Hypereosinophilic syndrome (HES) is a rare blood disorder. It occurs when a person’s blood has very high numbers of eosinophils. This is a type of white blood cell that plays an important role in the immune system. These eosinophils get into various tissues, causing inflammation and eventually organ dysfunction. The most commonly involved organs in HES include the skin, lungs, heart and the nervous system.
Most people have less than 500 eosinophils/microliter in their blood. People with HES usually have more than 1500 eosinophils/microliter in their blood for 6 months or more, and the cause cannot be identified.