Eosinophils are a type of white blood cell that has coarse granules within its cytoplasm. Eosinophils are produced in the bone marrow and migrate to tissues throughout the body. When a foreign substance enters the body, other types of white blood cells (lymphocytes and neutrophils) release substances to attract eosinophils and then release toxic substances to kill the invader.


Eosinophils are tasked with the duty of fighting off parasites and infections that cause allergies. They 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.

What can cause a number of eosinophils to be too high or too low?

An increased number of eosinophils in the blood (eosinophilia) usually indicates the response of the body to parasites or an allergic reaction. Elevated eosinophil counts are also common in some diseases such as asthma, eczema, autoimmune diseases, hay fever and leukemia.

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.