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Eosinophil biology and causes of eosinophilia

Peter F Weller, MD, MACP
Amy D Klion, MD
Section Editors
Donald H Mahoney, Jr, MD
Bruce S Bochner, MD
Deputy Editors
Alan G Rosmarin, MD
Anna M Feldweg, MD


Eosinophils are predominantly tissue-dwelling cells whose functions in health are not entirely understood. Eosinophils in the peripheral blood or tissues can increase in a wide array of disease states, ranging in severity from mild to life-threatening, and as a result of several mechanisms. When activated, eosinophils are capable of releasing mediators and substances that can damage tissues and contribute to disease pathology.

Normal eosinophil biology, the mechanisms of eosinophilia, tissue damage by eosinophils, and the major causes of eosinophilia are discussed in this topic review. An approach to the patient with eosinophilia and the diagnosis and management of the hypereosinophilic syndromes are reviewed separately. (See "Approach to the patient with unexplained eosinophilia" and "Hypereosinophilic syndromes: Clinical manifestations, pathophysiology, and diagnosis" and "Hypereosinophilic syndromes: Treatment".)


Eosinophils are white blood cells (WBCs) of the granulocytic lineage, which also includes neutrophils and basophils [1]. The true physiologic function(s) of eosinophils remain largely unknown although they are likely involved in host immune response to infection, tissue remodeling, tumor surveillance, and maintenance of other immune cells [2]. Eosinophils develop and differentiate in the bone marrow under the influence of interleukin (IL)-5, IL-3, and granulocyte-macrophage colony-stimulating factor (GM-CSF) [3].

Normal levels — In peripheral blood, an absolute eosinophil count (determined by multiplying the total WBC count by the percentage of eosinophils) of 0 to 500/microL (<0.5 x 109/L) is typically considered normal. Defining the normal range by percentage of WBC can be problematic because differences in absolute neutrophil counts between ethnic groups can dramatically impact the percent of eosinophilia [4]. Blood eosinophil counts have been reported to vary within the same person at different times of day and on different days, both in individuals with eosinophilic disorders and in healthy volunteers [5-7]. However, results are inconsistent among studies, and the variability in counts is rarely large enough to impact care.

Eosinophils are primarily tissue-dwelling cells; they are several hundred-fold more abundant in tissues than in blood [8]. In healthy individuals, eosinophils can be found in the digestive tract (although not the esophagus), spleen, lymph nodes, thymus, mammary glands, and uterus [2]. Recruitment of eosinophils to these tissues is mediated mainly by eotaxins, a family of chemokines (ie, cytokines that stimulate cell migration) that bind to the eosinophil chemokine receptor CCR3.

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Literature review current through: Oct 2017. | This topic last updated: Nov 03, 2015.
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