Interleukin-5 (IL-5) is a glycoprotein with molecular weight 40 to 50 kDa that consists of homodimer. The murine cDNA encodes 133 amino acids with a signal sequence of 18 amino acids. The human cD NA codes for a protein with 134 amino acids in which the first 19 are the predicted signal sequence. There is 70% homology at the amino acid level between murine and human IL-5. IL-5 R alpha chain cDNA encodes p 60 that binds IL-5 with low affinity. The beta subunit of IL-5, GM-CSF, and IL-3 receptor complexes is shared and these receptors show some functional cross-reactivity. Both alpha and beta chains of IL-5 R belong to a cytokine receptor superfamily. It is important to note that whilst murine IL-5 causes the activation of B cells and eosinophil differentiation, human IL-5 only causes eosinophil differentiation. IL-5 plays an important role in eosinophilopoiesis in vivo as well as in vitro.