Summary

Eosinophils are mature myeloid-derived cells, defined by the presence of prominent secondary granules [1]. In peripheral blood, the normal upper limit of absolute eosinophil count (AEC) ranges from 350 to 500 cells/µL. Eosinophilia is defined as an AEC exceeding 500 cells/µL, while hypereosinophilia is characterized by an AEC greater than 1500 cells/µL confirmed on at least two separate occasions [1,2].

Hypereosinophilic syndrome (HES) is defined by the presence of hypereosinophilia accompanied by organ damage or dysfunction attributed to eosinophilic infiltration in the absence of an alternative identifiable cause [1].

Eosinophilia may arise from a broad spectrum of conditions and is generally categorized as either primary (clonal) or secondary (reactive). Primary eosinophilia typically results from myeloid malignancies, whereas secondary eosinophilia is most commonly associated with allergic disorders, helminth infections, drug hypersensitivity reactions, rheumatologic disorders, and, less frequently, paraneoplastic phenomenon [1,3].