Eosinophilia is defined as an increase in the peripheral blood eosinophil count. The upper limit of normal is usually taken as about 600/microliter, but it is lower (about 400/microliter) if people with minor allergies are excluded. There is no ethnic variation in the normal eosinophil count and no physiologic cause of an increased count. 
The most common cause of eosinophilia depends on socioeconomic and geographic factors. In the developing world the most common cause is parasitic infection (and parasitic infections result in the greatest number of cases worldwide), whereas in the developed world it is allergy. Uncommon and rare causes are important because eosinophilia may be the result of a serious underlying condition.
The terms eosinophilia and hypereosinophilia do not have precise definitions. Nevertheless, the higher the eosinophil count, the more urgent it is to determine the cause, first because the underlying disease may need treatment in its own right, and second because hypereosinophilia can lead to tissue damage, which can be life-threatening. If eosinophils are extensively degranulated, tissue damage is more likely because of the enzymes, cytotoxic proteins, and cytokines released.
Investigation of eosinophilia should follow the traditional model of clinical history, physical exam, and laboratory and other investigations, taking into account the likely causes in the individual patient.
Professor of Diagnostic Haematology
St Mary's Hospital Campus
Imperial College London
BJB is an author of a number of references cited in this topic.
Department of Haematology
St George's University Hospitals NHS Foundation Trust
JU is on Bayer, Vifor Pharma and Octapharma scientific advisory boards; Bayer, Octapharma, Shire, Vifor Pharma and Daiichi-Sankyo speaker bureaus; is involved in clinical trials associated with Portola, Bayer, Boehringer-Ingelheim and Daiichi-Sankyo; and has received travel support from Bayer.
Professor of Medicine
Department of Internal Medicine
Division of Immunology/Allergy Section
University of Cincinnati
JB is the editor of the Joint Task Force Practice Parameters on Urticaria.
Department of Molecular and Developmental Genetics
Katholieke Universiteit Leuven
JC is an author of a reference cited in this topic.
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