IL-5 is a homodimeric cytokine expressed by T-lymphocytes and NK cells that functions as a critical regulator of eosinophil biology and immune responses. Mechanistically, IL-5 exerts its effects through a heteromeric receptor composed of IL5RA and the common cytokine receptor subunit CSF2RB, activating intracellular kinases (LYN, SYK, JAK2) that propagate signals via RAS-MAPK and JAK-STAT5 pathways 123. Primary functions include promotion of eosinophil survival, differentiation, and chemotaxis, as well as B-cell activation and immunoglobulin production 45. Beyond eosinophils, IL-5 receptors are functionally expressed on human airway epithelial cells, enabling direct effects on airway inflammation 6. Recent evidence reveals IL-5 enhances mast cell survival and interferon responses during viral infection via an IL-5/EPAS1 axis, potentially linking elevated IL-5 to asthma exacerbations triggered by respiratory infections 7. Clinically, IL-5 is a major therapeutic target in severe eosinophilic asthma and eosinophilic granulomatosis with polyangiitis (EGPA), where monoclonal antibodies (mepolizumab, reslizumab, benralizumab) targeting IL-5 or its receptor effectively reduce eosinophil counts, control disease activity, and enable corticosteroid reduction 891011.