Eosinophils are white blood cells that are important in fighting off certain infections.  They can cause some of the symptoms of allergies, and are also involved in diseases such as asthma. Inside the eosinophil are packets (or granules) that contain chemicals that can cause swelling, allergic symptoms and sometimes scarring when they are released. The reaction to the release of these packets is a normal part of the bodies defence system.  However, in some patients with Hypereosinophilic syndrome (HE) or Chronic eosinophilic leukaemia (CEL) this normal process goes out of control, and damage can occur to some parts of the body, including the heart, the lungs, the skin, and the bowel…amongst others.

An increase in eosinophils or hypereosinophilia in the blood is called an eosinophilia; only very rarely is this due to CEL or HE. It is much more likely to be due to what is called a reactive cause of eosinophilia such as allergies (asthma, eczema, hayfever and allergies to other things) or parasitic infections (for example worms and some tropical diseases).  Sometimes other blood conditions such as lymphomas may be present.