The IL-7 cytokine plays a major role in the in vivo maintenance of polyclonal naive and memory T cells, positively regulating the survival, differentiation and proliferation of thymocyte and peripheral T lymphocyte populations. IL-7 exerts its biological function through the IL-7 receptor which is expressed on pre-B cells, thymocytes and bone marrow-derived macrophages. The IL-7 receptor is composed of an IL-7 receptor-specific chain and the IL-2 receptor γ chain common to the IL-2, IL-4,IL-7, IL-9 and IL-15 receptors. Soluble forms of the IL-7 Rα have been reported.