CD129 is known as the 57 kD IL-9 receptor. It is a member of the hematopoietin receptor superfamily. Although the alpha-chain of the receptor binds IL-9 with high affinity, interaction with the gamma-chain (CD132) of the IL-2 receptor is required for signaling. The IL-9 receptor is expressed at low levels on eosinophils, mast cells, macrophages, B lymphocytes, T lymphocytes, and erythroid progenitors. IL-9 receptor binding initiates STAT activation required for the proliferative and anti-apoptotic effects of this cytokine. In humans, signals from the IL-9 receptor appear to be critical for intrathymic T cell development. IL-9 binding has been shown to increase IL-5 receptor expression and promote survival in human eosinophils.