Interleukin-5 (IL-5), a cytokine produced predominantly by type 2 helper T cells (Th2) and group 2 innate lymphoid cells (ILC2s). IL-5 is the principal regulator of eosinophil biology. It binds to the IL-5 receptor (IL-5Rα/βc) on eosinophil precursors in the bone marrow, promoting their proliferation, differentiation, and release into circulation. It also enhances eosinophil activation, tissue migration, and survival at sites of inflammation.