IL-7 (Interleukin 7) is a 20kDa hematopoietic growth factor secreted by stromal cells residing in the bone marrow, thymus, spleen and gut. IL-7 plays a critical role during early T-cell development of naïve and memory CD8+ T cells, as well as during the proliferation and differentiation of B cells. Furthermore, IL-7 has been shown to play a key role in coreceptor reversal, a process which induces the expression of CD8 receptor in a T-cell that has begun differentiation as a CD4+ T cell. IL-7 belongs to a family of short-chain four-helical bundle cytokines. These family members signal through heterodimeric receptor complexes that contain the common cytokine receptor gamma-chain, and activate signaling pathways that promote cell proliferation and survival such as the PI3K, MAPK, and JAK/STAT. IL-7 has been implicated in the development of rheumatoid arthritis (RA), HIV, acute graft-vs-host disease (aGVHD) and X-linked severe combined immunodeficiency (XSCID). In RA, IL-7 activates the production of TNF-alpha by CD4+ T cells and macrophages within the synovial fluid, leading to the inflammation characteristic of RA. In patients with HIV, IL-7 homeostasis is impaired due to decreased expression of the IL-7R.  The loss of T-cell homeostasis induces the lymphopenic condition characteristic of an HIV infection. Furthermore, the T-cell cytotoxic regimen prior to bone marrow transplantation induces IL-7 secretion, stimulating growth of residual allo-reactive T cells. These allo-reactive T-cells recognize and attack the transplanted cells and host tissues which lead to the development of aGVHD. Additionally, IL-7 knockout studies in mice have demonstrated that IL-7 is crucial for T cell survival as absence of IL-7 leads to XSCID. The rodent IL-7 proteins have low homology to human IL-7; only 56-57% amino acids sequence identity.