Mature mouse interleukin 5 (IL-5) is a 12 kDa protein consisting of 113 amino acids and is processed from a 133 amino acid precursor. IL-5 is mainly generated in T cells as a disulfide-linked homodimeric 40-45 kDa glycoprotein. The homodimer is formed by two interchain disulfide bonds between Cys 44 and 86 of each chain. T cell derived IL-5 appears to be the main cytokine involved in the control of eosinophilia. The effects of IL-5 on eosinophils include promoting growth and differentiation, activating mature eosinophils and increasing the survival of these cells during parasitic infection. In mice, IL-5 acts on B cells to induce differentiation into Ig-producing cells. The critical role of the eosinophils in allergy indicates that IL-5 is a key cytokine in the development of allergic diseases such as asthma. There is 71% homology between human and mouse IL-5 and significant crossreactivity in functional assays. Murine IL-5 receptor (IL-5R) consists of an α and β polypeptide chain. The IL-5R α chain is a 60 kDa protein with a low binding affinity for IL-5. Association with the 130 kDa IL-5R β chain increases the binding affinity for IL-5 aprroximately 100-fold, although the β chain does not bind IL-5 by itself.