Interleukin 5 (IL-5) is a member of the interleukin family with length of 115 amino acids. Interleukins are a group of cytokines (secreted proteins / signaling molecules) that were first seen to be expressed by white blood cells (leukocytes) and has been found in a wide variety of body cells. Interleukin 5 or IL-5 is produced by T helper-2 cells and mast cells. It helps to stimulate B cell growth and increase immunoglobulin secretion and is considered as a key mediator in eosinophil activation. Interleukin 5 (IL-5) has long been associated with several allergic diseases, including allergic rhinitis and asthma. Growth in the number of circulating, airway tissue, and induced sputum eosinophils have been observed in patients with these diseases. IL-5 also had something with the terminally differentiated granulocyte eosinophils. IL-5 was originally found as an eosinophil colony stimulating factor. It has been proved to be a major regulator of eosinophil accumulation in tissues, and can modulate eosinophil behavior at every stage from maturation to survival.