IL-2 is amember of a family of cytokines, which also includes IL-4, IL-7, IL-9, IL-15and IL-21. IL-2 signals through a receptor complex consisting of IL-2specific IL-2 receptor alpha (CD25), IL-2 receptor beta (CD122) and a commongamma chain (γc), which is shared by all members of this family of cytokines.Binding of IL-2 activates the Ras/MAPK, JAK/Stat and PI 3-kinase/Aktsignaling modules.<br/>IL-2 isnormally produced by the body during an immune response. When environmentalsubstances (molecules or microbes) gain access to the body, these substancesare recognized as foreign by antigen receptors that are expressed on thesurface of lymphocytes. Antigen binding to the T cell receptor (TCR)stimulates the secretion of IL-2, and the expression of IL-2 receptors IL-2R.The IL-2/IL-2R interaction then stimulates the growth, differentiation andsurvival of antigen-selected cytotoxic T cells via the activation of theexpression of specific genes. As such, IL-2 is necessary for the developmentof T cell immunologic memory, one of the unique characteristics of the immunesystem, which depends upon the expansion of the number and function ofantigen-selected T cell clones.<br/>IL-2 isalso necessary during T cell development in the thymus for the maturation ofa unique subset of T cells that are termed regulatory T cells (Tregs). Afterexiting from the thymus, T-Regs function to prevent other T cells fromrecognizing and reacting against "self antigens", which couldresult in "autoimmunity". T-Regs do so by preventing the respondingcells from producing IL-2. Thus, IL-2 is required to discriminate betweenself and nonself, another one of the unique characteristics of the immunesystem.<br/>IL-2 hasbeen found to be similar to IL-15 in terms of function. Both cytokines areable to facilitate production of immunoglobulins made by B cells and inducethe differentiation and proliferation of natural killer cells. The primarydifferences between IL-2 and IL-15 are found in adaptive immune responses.For example, IL-2 participates in maintenance of T-Regs and reducesselfreactive T cells. On the other hand, IL-15 is necessary for maintaininghighly specific T cell responses by supporting survival of CD8 memory Tcells.