Human IL-4 exists in molecular weight forms between 15 and 19 kDa, representing variable glycosylation. The interleukin 4 (IL-4) is a cytokine that induces differentiation of naive helper T cells (Th0 cells) to Th2 cells. Upon activation by IL-4, Th2 cells subsequently produce additional IL-4 in a positive feedback loop. The cell that initially produces IL-4, thus inducing Th0 differentiation, has not been identified, but recent studies suggest that basophils may be the effector cell. It is closely related and has functions similar to IL-13.
IL-4 also has been shown to drive mitogenesis, dedifferentiation,and metastasis in rhabdomyosarcoma. IL-4, along with other Th2 cytokines, is involved in the airway inflammation observed in the lungs of patients with allergic asthma.