IL-5 is a 115-amino acid-long Th2 cytokine that is part of the hematopoietic family. Unlike other members of this cytokine family (namely IL-3 and GM-CSF), this glycoprotein in its active form is a homodimer. IL-5 is produced by Th2 cells and mast cells. The spectrum of biological
activities of IL-5, on B cells as well as other cell types suggests that IL-5 plays an important functional role in the activities of Th2 cells. Through binding to the IL-5 receptor, IL-5 stimulates B cell growth and increases immunoglobulin secretion. It is also a key mediator in eosinophil activation.
IL-5 has long been associated with the cause of several allergic diseases including allergic rhinitis and asthma, wherein a large increase in the number of circulating, airway tissue, and induced sputum eosinophils have been observed.