Eosinophil Biology Today—A Primer for Basic and Clinical Investigators

Eosinophils are multifunctional granulocytes that participate in tissue homeostasis, host defence, inflammation, and repair. Their activities are now known to extend beyond type-2 immunity and include the release of diverse cytokines and growth factors, such as IL-4, IL-13, TGF-β, IL-1β, GM-CSF, and TNF-α, together with context-dependent immunomodulatory, cytotoxic, and pro-remodelling functions. Recent advances highlight the importance of tissue imprinting and microenvironmental cues in shaping eosinophil phenotypes, revealing substantial functional plasticity and transcriptional diversity across physiological and pathological settings. Here we synthesise essential concepts in eosinophil biology and provide an overview of the most widely used approaches for visualising, isolating, and functionally characterising these cells, emphasising methodological strengths, limitations, and common artefacts. We further outline how transcriptomic and proteomic tools have refined the understanding of eosinophil phenotypes and their relevance to disease, including allergy, infection, tissue repair, and cancer. Overall, we provide a resource for basic and clinical investigators who are not currently working in eosinophil biology, but might be attracted to this multidisciplinary area in view of many recent exciting developments.