Eosinophil diversity and function: emerging insights into subtypes and therapeutic targets

Eosinophils are multifunctional polymorphonuclear granulocytes garnering attention for their roles in disease and maintaining physiological homeostasis. Historically, these immune cells were known to mediate allergic responses and defend against parasitic infections. Recent evidence further links eosinophils to several eosinophil-associated disorders, including eosinophilic esophagitis, eosinophilic granulomatosis with polyangiitis, and hypereosinophilic syndrome; however, their pathogenic contribution appears to be disease-specific and remains incompletely defined in some contexts. The advances in immunology and molecular biology have yielded targeted biologic therapies against eosinophils, significantly improving outcomes for patients with eosinophilic disorders. Concurrently, research reveals that eosinophils are a heterogeneous cell population comprising distinct subtypes that differ in surface protein expression, tissue localization, and functional properties. Specifically, resident eosinophils (rEos) reside in tissues for local maintenance and immune regulation, whereas inflammatory eosinophils (iEos) are rapidly recruited during immune responses to drive inflammation. Initially characterized in murine models, similar eosinophil heterogeneity has since been confirmed in human blood and tissues. Recognizing distinct eosinophil subsets is crucial for understanding their contributions to disease progression. This review highlights recent discoveries in eosinophil diversity and explores therapeutic implications of selectively targeting specific eosinophil subsets in clinical practice.