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Quick Facts

An eosinophil is a granular leukocyte with a nucleus that usually has two lobes connected by a slender thread of chromatin, and cytoplasm containing coarse, round granules that are uniform in size (Dorland, 2011).

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Cell Morphology

Eosinophils are one type of leukocyte and account for only 1–4% of all leukocytes. There are around 100–400 eosinophils per microliter of blood. They are relatively the same size as neutrophils with a diameter of 12–15 μm and lifespan of around 5 days (Pawlina, 2016; Marieb, Wilhelm and Mallatt, 2012).

Eosinophils have bilobed nuclei and contain multiple large cytoplasmic granules. These granules contain digestive enzymes that are released during an allergic reaction or parasitic infection.

Eosinophils are predominately aggregated in the digestive tube, where parasites are more likely to be encountered. Eosinophils are also commonly found at sites of chronic inflammation.


Eosinophils develop in the red bone marrow from myeloid stem cells that arise from hematopoietic stem cells and development usually takes 7–11 days (Marieb, Wilhelm and Mallatt, 2012). When eosinophils are fully developed, they are released into the blood and circulate there for around 10 hours, before migrating to connective tissue (Eroschenko, 2008).

The number of eosinophils increases when the body is invaded by parasites or when an individual has an allergy. Production of eosinophils is driven by growth factors such as granulocyte-stimulating factor.


Eosinophils provide defense against helminthic parasites by phagocytosis and releasing digestive enzymes. These digestive enzymes digest and destroy the parasites.

Eosinophils end an allergic reaction by secreting an enzyme called histaminase which degrades histamine, and by releasing chemicals that neutralize inflammatory mediators (Marieb, Wilhelm and Mallatt, 2012; Pawlina, 2016).

List of Clinical Correlates




Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Eroschenko, V. P. (2008) DiFiore's Atlas of Histology with Functional Correlations. Wolters Kluwer Health/Lippincott Williams & Wilkins.

Marieb, E. N., Wilhelm, P. B. and Mallatt, J. (2012) Human Anatomy. 14th edn.: Benjamin Cummings.

Pawlina, W. 2016. Histology: A text and atlas with correlated cell and molecular biology. 7th ed. Philadelphia: Wolters Kluwer.

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