There are several distinct differences in the functions of IL-2 and IL-15. IL-2 is involved in checkpoints or brakes on the immune system. IL-2 is required to maintain the competitiveness of forkhead box P3- (FoxP3-) expressing regulatory T cells, and plays a crucial role in activation-induced cell death (AICD), a process that leads to the elimination of self-reactive T cells. Thus, the unique feature of IL-2 is to prevent T cell immune response to self that would lead autoimmunity. In contrast, IL-15 expression has no effect on regulatory T cells but is an anti-apoptotic factor in several systems. Furthermore, IL-15 promotes the maintenance of CD8+ CD44 memory T cells. Thus, IL-15 has its primary role the maintenance of a robust memory response to invading pathogens.