Interleukin 5 (IL-5) is a kind of peptide hormone released from T lymphocytes of mammals infected with microorganisms or parasites. It is an acidic glycoprotein with a molecular mass of 40 to 50 kDa that consists of a homodimer of polypeptides. It controls hematopoiesis so that it increases natural immunity. In the mouse, IL-5 acts on committed B cells to induce differentiation into Ig-producing cells and on common progenitors for CD5+ pre-B cells and CD5+ macrophages to support their survival. The antibodies secreted by CD5+ B cells seem to be responsible for the primary protection against the infection with microorganisms or parasites. It also supports the growth and/or differentiation of eosinophil precursor and mature eosinophils, which can be effective for the removal of parasites in combination with the antibodies against them. Murine 1123 receptor (IL-5R) consists of two different polypeptide chains; cc chain and p chain. The IL-5R a chain is 60 kDa protein that binds IL-5 with low affinity. The IL-SR p chain is a 130 kDa protein which does not bind IL-5 by itself but is necessary to form the high affinity IL-5R. The p chain was identified by using one of the anti-ILdR mAb and anti-IL-3R mAb as the IL3R homologue. This p chain is also used as the p chain of GM-CSF receptor. This fact suggests that there is a common signaling mechanism among these cytokines and efficient cooperation among them. At the same time, these findings may explain the overlapping role of these cytokines in the development of granulocytes.