IL-7 (Interleukin-7) is a cytokine that plays important roles in lymphocyte differentiation, proliferation, and survival. IL-7 is produced by stromal epithelial cells of the thymus, bone marrow, and intestines. It signals through a receptor complex composed of IL-7 R alpha/CD127 and the Common gamma Chain. gamma c is also a subunit of the receptors for IL-2, -4, -9, -15, and -21. IL-7 contributes to the maintenance of all naïve and memory T cells. It is required for optimal T cell-dendritic cell interaction. In mouse, IL-7 activation of IL-7 R alpha is critical for both T cell and B cell lineage development, while in humans, it is required for T cell but not for B cell development.