Interleukin-5, or IL-5, was originally discovered as a soluble T cell-derived factor, called T cell-replacing factor (TRF), that induced T cell-depleted activated B cells to secrete immunoglobulin. IL-5 is a key hematopoietic cytokine in eosinophil differentiation, maturation, recruitment and activation at sites of allergic inflammation. IL-5 also plays a role in the development, metabolism, and function of basophils. IL-5 exerts its biological activity through the IL-5 receptor (IL-5R), which is composed of at least two chains: an α chain that binds IL-5 with low affinity and a βchain that does not bind IL-5, but together with the IL-5α chain, constitutes the high affinity IL-5 receptor. The β chain is common to the IL-3, IL-5 and Gm-csf receptors and has been shown to signal through the JAK/Stat pathway.