Hypereosinophilia is a disease characterised by a marked increase in the eosinophil count in the bloodstream.

Eosinophils are a type of white blood cell produced in the bone marrow and released into the circulatory system. Eosinophils are only a small minority of white blood cells and are found in the tissues of the lung and gastro-intestinal tract. Under normal circumstances, the eosinophil count in the blood is about 0.4 x 109/L (0.1 - 0.6).

Eosinophils perform a vital protective function in the body. When the body is faced with invasive organisms or allergens, they travel to the bloodstream and release toxins that destroy the invaders. When fighting infection, eosinophil counts are higher than normal. In such patients, the toxins released by the increased eosinophils start to affect organ function. The commonest cause of eosinophilia worldwide is infection, and in industrialized nations, atopic disease.

Clonal bone marrow disorders such as leukemia and lymphoma can also cause eosinophilia, but generally account for less than 1% of eosinophilic syndrome.

However, in some people, high eosinophil counts can persist for long periods, even without infection, allergic or clonal causes. Such cases are diagnosed as Idiopathic Hypereosinophilic Syndrome (HES). This is a very rare condition. In 25% of cases previously diagnosed as HES a clone of T cells can now be demonstrated.

Whatever the cause for the eosinophilias, in certain circumstances the eosinophils produce damage to various organs (eosinophil end-organ damage) by activation of eosinophils, thrombotic events, release of eosinophil granule contents and deposition of eosinophil proteins.

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