IL-7 is a member of the type I cytokine family. The primary sources of IL-7 are non-hematopoietic stromal cells in bone marrow, thymus, and lymphoid organs and tissues. It is a pleiotropic cytokine with central roles in modulating T cell development and peripheral T cell homeostasis. IL-7 can act both as a T cell growth factor as well as a critical anti-apoptotic survival factor for naive and memory T cells. IL-7 is related to IL-2 and signals through a heterodimeric receptor composed of the common cytokine signaling γ-chain and IL-7 receptor α-chain.