Understanding Eosinophil Heterogeneity: The Known and Unknown.

Eosinophils are multifunctional granulocytes with central roles in the pathobiology of chronic airway diseases. While systemic eosinophilia (>300 cells/μL) is a well-established biomarker to guide therapeutic decision-making, accumulating evidence indicates that eosinophils are not a uniform population but instead exhibit substantial phenotypic and functional heterogeneity across biological compartments, inflammatory states, and disease contexts. In this review, we synthesize the current understanding of eosinophil heterogeneity in airway diseases and critically evaluate the strengths and limitations of surface marker-based approaches, with emphasis on CD62L/L-selectin-defined subpopulations. Although CD62L-based stratification has provided valuable insight into eosinophil activation and tissue localization, its limited specificity, inconsistent clinical associations, and reliance on murine models restrict its utility as a framework for eosinophil subtyping in humans. We highlight how transcriptomic and proteomic profiling has transformed the field by revealing that peripheral blood eosinophils are largely quiescent, whereas disease-relevant functional specialization is predominantly acquired within inflamed tissues in response to cues from the local microenvironment. These molecular studies support a model in which eosinophil heterogeneity represents a continuum of activation rather than discrete, fixed subsets. A refined, integrative approach to understanding eosinophil heterogeneity is critical for improving patient stratification, predicting therapeutic responsiveness, and optimizing precision medicine strategies in chronic airway diseases.