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A lesson from the wild: The natural state of eosinophils is Ly6G hiMair, Iris; orcid: 0000-0002-7326-3114; email: email@example.com; Wolfenden, Andrew; Lowe, Ann E.; Bennett, Alex; orcid: 0000-0003-4869-9132; Muir, Andrew; orcid: 0000-0002-1461-8712; Smith, Hannah; orcid: 0000-0001-8911-9048; Fenn, Jonathan; orcid: 0000-0002-9255-6975; Bradley, Janette E.; orcid: 0000-0003-3973-7977; email: firstname.lastname@example.org; Else, Kathryn J.; orcid: 0000-0001-6660-055X; email: Kathryn.email@example.com (2021-09-15)Abstract: With a long history of promoting pathological inflammation, eosinophils are now emerging as important regulatory cells. Yet, findings from controlled laboratory experiments so far lack translation to animals, including humans, in their natural environment. In order to appreciate the breadth of eosinophil phenotype under non‐laboratory, uncontrolled conditions, we exploit a free‐living population of the model organism Mus musculus domesticus. Eosinophils were present at significantly higher proportions in the spleen and bone marrow of wild mice compared with laboratory mice. Strikingly, the majority of eosinophils of wild mice exhibited a unique Ly6Ghi phenotype seldom described in laboratory literature. Ly6G expression correlated with activation status in spleen and bone marrow, but not peritoneal exudate cells, and is therefore likely not an activation marker per se. Intermediate Ly6G expression was transiently induced in a small proportion of eosinophils from C57BL/6 laboratory mice during acute infection with the whipworm Trichuris muris, but not during low‐dose chronic infection, which better represents parasite exposure in the wild. We conclude that the natural state of the eosinophil is not adequately reflected in the standard laboratory mouse, which compromises our attempts to dissect their functional relevance. Our findings emphasize the importance of studying the immune system in its natural context – alongside more mechanistic laboratory experiments – in order to capture the entirety of immune phenotypes and functions.