Interleukin 5 (IL-5) ligand belongs to the cytokine superfamily. IL-5 is a glycoprotein which belongs to the cytokine superfamily. It possesses the four helical bundle motifs that is conserved among several hematopoietic cytokines. IL-5 plays an important role in the proliferation and differentiation of eosinophils. IL-5 induces terminal maturation of eosinophils, prolongs eosinophils survival by delaying apoptotic death, increases eosinophils adhesion to endothelial cells and enhances eosinophils effector function. IL-5 plays important roles in the pathogenesis of asthma, hypereosinophilic syndromes and eosinophil-dependent inflammatory diseases. IL-5 is produced by eosinophils, mast cells, Th2 cells, Tc2 cells and gamma delta T cells. IL-5 exerts influence on different biological activities by associating with the IL-5 receptor.