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. Both human and mouse IL-5 are glycosylated disulfide-linked homodimers. The IL-5 receptor is a heterodimer that consists of a high affinity IL-5 binding α chain and the common β chain, which is shared by GM-CSF and IL-3 receptors, for signal transduction. IL-5 is initially synthesized as a precursor with a 19 amino acid signal peptide which is cleaved to form a 112 amino acid mature protein. IL-5 induces eosinophil activation, proliferation, and differentiation in both mice and humans.
