Purity >95% as determined by SDS-PAGE. Interleukin 5 (IL-5) is a hematopoietic growth factor that is expressed in type 2 T helper (Th2) cells, mast cells, and eosinophils. IL-5 acts through the IL-5 receptor (IL-5R), stimulates B cell growth, and mediates eosinophil activation. IL-5 expression is regulated by the GATA binding protein 3 (GATA3) transcription factor. Human and mouse IL-5 show cross-reactivity.