Since the discovery of eosinophils in the late 18th century, there has been an active debate surrounding the role of eosinophils in human health and disease. While eosinophils represent less than 5% circulating blood leukocytes in a healthy adult, eosinophil numbers can increase dramatically in both blood and/or tissue during various inflammatory processes, including infections (parasitic helminth, fungal, viral, and even selected bacterial), non-specific tissue injury, malignancy, and, in particular, allergic diseases. Recent findings indicate that not only Th2 cells, but also type 2 innate lymphoid cells, mediate IL-5 production leading to eosinophilia. When viewed as exclusively end-stage effector cells, it has been stressed that eosinophils are an important source of toxic granule proteins, mediators, cytokines, and reactive oxygen species. However, eosinophils have immunoregulatory capacities and can be either beneficial or harmful depending on local conditions, with involvement in numerous biological processes, such as reproduction, metabolism, and innate/adaptive immunity.

Severe eosinophilic disorders present a significant clinical challenge. This is due to several factors, including the varied etiologies, a high tendency to recur, and a frequent need to treat with long-term systemic steroids, accompanied by a host of short- and long-term side effects. Eosinophilic disorders can in certain circumstances be life threatening. Lessons from the recent emergence of anti-IL-5 monoclonal antibody therapies have highlighted the clinical benefits of eosinophil depletion.

To promote improved understanding of the eosinophil, a pleiotropic multifunctional leukocyte, and associated eosinophilic diseases, potentially leading to advances in clinical management, we seek articles from basic science to clinical investigations that cover, but are not limited to:

1. Physiological roles of eosinophils.
2. Immuno-regulatory properties of eosinophils.
3. Eosinophilic lung diseases (e.g. allergic bronchopulmonary aspergillosis/mycosis, eosinophilic asthma, aspirin-induced asthma, eosinophilic pneumonias).
4. Eosinophilic gastrointestinal diseases.
5. Eosinophilic granulomatosis with polyangitis (EGPA).
6. Eosinophilic otological diseases (e.g. allergic fungal sinusitis, eosinophilic otitis media, chronic rhinosinusitis with nasal polyps, eosinophilic chronic rhinosinusitis).
7. Eosinophilic skin and subcutaneous diseases.
8. Eosinophilic heart diseases.
9. Hypereosinophilic syndromes.
10. Eosinophilic parasitic diseases.

We welcome the submission of Original Research, Review and Opinion articles, Methods, and Case reports which address relevant insights into the field in this Research Topic. Basic studies on eosinophils and eosinophil physiology may also be included.

Keywords: Eosinophils, Eosinophilic disorders

Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.