IL-2 was discovered through its function as a T cell growth factor (TCGF), and plays a pivotal role in immune responses against pathogenic infection. Recognition and binding of the foreign Ags by the TCRs stimulate both the secretion of IL-2 and the expression of IL-2Rs on the T cell surface. Subsequently, the IL-2/IL-2R interaction activates the intracellular Ras/Raf/MAPK, JAK/STAT, and PI3K/AKT signal pathways, and ultimately stimulates the growth, differentiation, and survival of the Ag-selected cytotoxic T cells. Human IL-2 acts on murine and human T cells, and its receptors are shared by others cytokines. IL-2Rα is an IL-2–specific receptor, IL-2Rβ is shared with IL-15 and the γ chain that is a common receptor shared by many cytokines including IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21.