Interleukin 7 Receptor alpha (IL-7 R alpha ), also known as CD127, is a 75 kDa hematopoietin receptor superfamily member that plays an important role in lymphocyte differentiation, proliferation, and survival. Mature human IL-7 R alpha consists of a 219 amino acid (aa) extracellular domain (ECD) with one fibronectin type-III domain and a WSXWS motif, a 25 aa transmembrane segment, and a 195 aa cytoplasmic domain. Alternate splicing of human IL-7 R alpha generates a secreted soluble form of the receptor. Within the ECD, human IL‑7 R alpha shares 67% aa sequence identity with mouse and rat IL-7 R alpha. IL-7 R alpha associates with the common gamma chain ( gamma c) to form the functional high affinity IL-7 receptor complex. The gamma c is also a subunit of the receptors for IL-2, -4, -9, -15, and -21. IL-7 R alpha additionally associates with TSLP R to form the functional receptor for thymic stromal lymphopoietin (11, 12). TSLP indirectly regulates T cell development by modulating dendritic cell activation. Knockout of TSLP R in mice provokes minor changes in B and T cell development compared to those seen with IL-7 R alpha deletion. The complexity of IL-7 R alpha biology is suggested by the competition between IL-7 and TSLP for receptor binding and by the ability of IL-7 R alpha to form functional complexes with SCF R and HGF R.