Interleukin 5 (IL5) is a specific hematopoietic growth factor that is responsible for the growth and differentiation of eosinophils. IL5 promotes the growth of immature hematopoietic progenitor cells BFU-E while it causes differentiation of CFU-E the proliferation of which is inhibited by IL5. B cells can be made responsive to IL5 by treatment with suboptimal doses of IL1. Interleukin 5 also promotes the generation of cytotoxic T cells from thymocytes. In thymocytes IL5 induces the expression of high affinity IL2 receptors. Human and mouse IL5 is cross-species reactive. Recombinant Human IL5 is a 26.5 kDa disulfide-linked homodimerized protein.