Eosinophils in solid cancers: sentinels, predictors, and therapeutic allies

Eosinophils are multifunctional granulocytes traditionally recognized for their roles in allergic responses and defense against parasitic infections. However, their involvement in cancer biology has emerged as an area of research interest in recent years. Epidemiological studies consistently suggest a protective role against tumor development, while in established malignancies, their prognostic significance varies considerably between cancer types. The most compelling evidence for clinical utility has emerged in cancer immunotherapy, where both baseline and treatment-induced eosinophil elevations consistently correlate with improved checkpoint inhibitor responses, enhanced survival outcomes, but also increased immune-related adverse events across multiple tumor types. Mechanistic studies reveal dual anti- and protumoral functions mediated through direct cytotoxic effects via granule proteins and indirect immune microenvironment modulation. This review synthesizes current knowledge on eosinophils in cancer biology and discusses the translational challenges that must be addressed to harness their clinical potential as biomarkers and therapeutic targets in oncology.