Eosinophils usually account for less than 7% of the circulating white blood cells (100 to 500 eosinophils per microliter of blood). These cells have a role in the protective immunity against certain parasites but also contribute to the inflammation that occurs in allergic disorders.

An increased number of eosinophils in the blood (eosinophilia) usually indicates the response of the body to parasites or an allergic reaction.

A low number of eosinophils in the blood (eosinopenia) can occur with Cushing syndrome, stress reactions, and treatment with corticosteroids but does not usually cause problems because other parts of the immune system compensate adequately.

Idiopathic hypereosinophilic syndrome: Idiopathic hypereosinophilic syndrome is an uncommon disorder in which the number of eosinophils increases to more than 1,500 cells per microliter of blood for more than 6 months without an obvious cause.

People of any age can develop idiopathic hypereosinophilic syndrome, but it is more common in men older than 50. The increased number of eosinophils can damage the heart, lungs, liver, skin, and nervous system. For example, the heart can become inflamed in a condition called Löffler endocarditis, leading to formation of blood clots, heart failure, heart attacks, or malfunctioning heart valves.

Symptoms may include weight loss, fevers, night sweats, fatigue, cough, chest pain, swelling, stomachache, rashes, pain, weakness, confusion, and coma. Additional symptoms of this syndrome depend on which organs are damaged. The syndrome is suspected when repeated blood tests reveal that the number of eosinophils is persistently increased in people who have these symptoms. The diagnosis is confirmed when doctors determine that the eosinophilia is not caused by a parasitic infection, an allergic reaction, or another diagnosable disorder.

Without treatment, generally more than 80% of the people who have this syndrome die within 2 years, but with treatment, more than 80% survive. Heart damage is the principal cause of death. Some people need no treatment other than close observation for 3 to 6 months, but most need drug treatment with prednisone or hydroxyureaSome Trade Names
HYDREA
. Some people with idiopathic hypereosinophilic syndrome have an acquired abnormality of a gene that regulates cell growth. This type of hypereosinophilia can respond to treatment with imatinibSome Trade Names
GLEEVEC
, a drug used to treat cancer. If these drugs fail, various other drugs may be used, and they can be combined with a procedure to remove eosinophils from the blood (apheresis).

Last full review/revision January 2013 by Mary Territo, MD