IL-2 and its receptor have key roles in key functions of the immune system, tolerance and immunity, primarily via their direct effects on T cells. In the thymus, where T cells mature, they prevent autoimmune diseases by promoting the differentiation of certain immature T cells into regulatory T cells, which kill off other T cells that are primed to attack normal healthy cells in the body. IL-2/IL2R also promotes the differentiation of T cells into effector T cells and into memory T cells when the initial T cells is also stimulated by an antigen, thus helping the body fight off infections. Through their role in the development of T cell immunologic memory, which depends upon the expansion of the number and function of antigen-selected T cell clones, they also have a key role in enduring cell-mediated immunity.