Interleukin 5 (IL-5) is a member of the hematopoietic family of Th2 cytokines and consists of 115 amino acids. Unlike other members of this cytokine family (IL-3 and GM-CSF), the activated form of this glycoprotein is a homodimer.IL-5 is secreted by Th2 cells and mast cells. The biological activity it produces on B cells, as well as other cell types, suggests that IL-5 has a functionally important role in the activity of Th2 cells.IL-5 stimulates B cell growth and enhances immunoglobulin secretion by binding to its receptor. It is also a key factor in the activation of eosinophils.IL-5 has long been associated with a variety of metaplastic diseases, including allergic rhinitis and asthma, where a large increase in IL-5 and a decrease in sputum eosinophils have been observed in circulating, tracheal tissues.