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These [[cell (biology)|cell]]s are [[eosinophilic]] or "[[acid]]-loving" due to their large acidophilic cytoplasmic granules, which show their affinity for acids by their affinity to [[coal tar dye]]s: Normally [[transparency (optics)|transparent]], it is this affinity that causes them to appear brick-red after [[staining]] with [[eosin]], a red [[dye]], using the [[Romanowsky stain|Romanowsky method]].<ref name="Foster">{{cite journal | vauthors = Rosenberg HF, Phipps S, Foster PS | title = Eosinophil trafficking in allergy and asthma | journal = The Journal of Allergy and Clinical Immunology | volume = 119 | issue = 6 | pages = 1303–10; quiz 1311–2 | date = June 2007 | pmid = 17481712 | doi = 10.1016/j.jaci.2007.03.048 | hdl = 1885/30451 | hdl-access = free }}</ref> The staining is concentrated in small [[Granule (cell biology)|granules]] within the cellular [[cytoplasm]], which contain many chemical mediators, such as [[eosinophil peroxidase]], [[ribonuclease]] (RNase), [[deoxyribonuclease]]s (DNase), [[lipase]], [[plasminogen]], and [[major basic protein]]. These mediators are released by a process called [[degranulation]] following activation of the eosinophil, and are [[toxin|toxic]] to both parasite and host tissues.
In normal individuals, eosinophils make up about 1–3% of white blood cells, and are about 12–17 [[micrometre]]s in size with bilobed nuclei.<ref name="Uhm">{{cite journal | vauthors = Uhm TG, Kim BS, Chung IY | title = Eosinophil development, regulation of eosinophil-specific genes, and role of eosinophils in the pathogenesis of asthma | journal = Allergy, Asthma & Immunology Research | volume = 4 | issue = 2 | pages = 68–79 | date = March 2012 | pmid = 22379601 | pmc = 3283796 | doi = 10.4168/aair.2012.4.2.68 }}</ref><ref>{{Cite book | last1 = Young | first1 = Barbara | last2 = Lowe | first2 = joseph o'connell | last3 = Stevens | first3 = Alan | last4 = Heath | first4 = John W. | name-list-style = vanc | title = Wheater's Functional Histology | publisher = Elsevier Limited | year = 2006 | edition = 5th | isbn = 978-0-443-06850-8 }}</ref> While eosinophils are released into the bloodstream, they reside in tissue.<ref name="Foster" /> They are found in the [[medulla oblongata|medulla]] and the junction between the [[Cortex (anatomy)|cortex]] and medulla of the [[thymus]], and, in the lower [[gastrointestinal]] tract, [[ovary|ovaries]], [[uterus]], [[spleen]], [[prostate]], and [[lymph nodes]], but not in the [[lung]]s, [[skin]], [[esophagus]], or some other internal organs{{Vague|date=September 2009}} under normal conditions. The presence of eosinophils in these latter organs is associated with disease. For instance, patients with eosinophilic asthma have high levels of eosinophils that lead to inflammation and tissue damage, making it more difficult for patients to breathe.<ref name="Lambrecht">{{cite journal | vauthors = Lambrecht BN, Hammad H | title = The immunology of asthma | journal = Nature Immunology | volume = 16 | issue = 1 | pages = 45–56 | date = January 2015 | pmid = 25521684 | doi = 10.1038/ni.3049 | s2cid = 5451867 }}</ref><ref name="Sanderson">{{cite journal |last1=Sanderson |first1=Colin |title=Interleukin-5, Eosinophils, and Disease |journal=Blood |date=1992 |volume=79|issue=12|pages=3101–3109|doi=10.1182/blood.V79.12.3101.bloodjournal79123101 |pmid=1596561 |doi-access=free }}</ref> Eosinophils persist in the circulation for 8–12 hours, and can survive in tissue for an additional 8–12 days in the absence of stimulation.<ref>{{Cite book | last1 = Young | first1 = Barbara | last2 = Lowe | first2 = James S. | last3 = Stevens | first3 = Alan | last4 = Heath | first4 = John W. | name-list-style = vanc | title = Wheater's Functional Histology | publisher = Elsevier Limited | year = 2006 | edition = 5th | isbn = 978-0-443-06850-8 }}</ref> Pioneering work in the 1980s elucidated that eosinophils were unique granulocytes, having the capacity to survive for extended periods of time after their maturation as demonstrated by ex-vivo culture experiments.<ref>{{cite journal | vauthors = Park YM, Bochner BS | title = Eosinophil survival and apoptosis in health and disease | journal = Allergy, Asthma & Immunology Research | volume = 2 | issue = 2 | pages = 87–101 | date = April 2010 | pmid = 20358022 | pmc = 2846745 | doi = 10.4168/aair.2010.2.2.87 }}</ref>
==Development==
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