With a diameter of 16 µm, eosinophil granulocytes are round and slightly larger than neutrophil granulocytes. The granules are coarse, bright red to honey-yellow and very densely packed. The nucleus is usually bilobed.

Normal range:
Eosinophils are rarely found in a blood film and have normal values of  - x 109/L. An increase of the eosinophil count is called eosinophilia and a decrease eosinopenia.

The eosinophils play an important role in allergic diseases as well as in parasitic infections. Like neutrophils, they are capable of phagocytosis and migration. By degranulation, the content of the eosinophil granules (peroxidase, leukotriens, platelet activating factor, cytokines) is released in order to fight bigger organisms such as parasites. Thus, eosinophils are capable of immunomodulation.

Clinical Significance:
Eosinophilia is usually caused by allergic or parasitic diseases. Many medications can cause eosinophilia as well.  Neoplastic diseases (e.g. Hodgkins disease) are occasionaly accompied by eosinophilia. Cases in which values of  > 1.5 x 109/L persist, and a cause cannot be established, are refered to as idiopathic eosinophilia syndromes.
Eosinopenia is commonly seen in situations of stress and acute infections. Earlier, reoccurance of eosinophils was considered as sign of recovery from an infectious disease.

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