IL-2 is a pleiotropic cytokine that plays a central role in immune responses. It promotes CD8+ T cell and NK cell cytolytic activity, modulates T cell differentiation, and is essential for the development and maintenance of T regulatory (Treg) cells. IL-2 also mediates tolerance by promoting activation-induced cell death (AICD). This review discusses the molecular mechanisms and complex cellular actions of IL-2, its interactions with other cytokines, and its use in clinical medicine. IL-2 is a 15.5 kDa type 1 cytokine produced primarily by CD4+ T cells, and it binds to three classes of IL-2 receptors: IL-2Rα (CD25), IL-2Rβ (CD122), and IL-2Rγ (CD132). These receptors have different affinities for IL-2, with the high-affinity receptor being essential for signaling. IL-2 signaling is mediated through the Janus family tyrosine kinases JAK1 and JAK3, which activate STAT5, leading to the transcription of genes involved in T cell proliferation and differentiation. IL-2 also plays a critical role in T helper cell differentiation, including Th1, Th2, Th17, and Tfh cells. It promotes Th1 and Th2 cell differentiation while inhibiting Th17 cell differentiation. IL-2 is essential for the development of Treg cells and for maintaining immune homeostasis. In addition, IL-2 is involved in the regulation of autoimmune diseases and has been used as a therapeutic agent in clinical settings. However, its use is limited by its toxicity, including severe capillary leak syndrome. Recent advances in IL-2 therapy include the development of super-IL-2 variants that exhibit enhanced binding to IL-2Rβ and do not require IL-2Rα. These variants induce greater expansion of cytotoxic T cells but less expansion of Treg cells. In addition, inhibitors of IL-2 signaling, such as JAK inhibitors, have been developed as potential therapeutic agents. These inhibitors block the signaling pathways of IL-2 and other cytokines that utilize the inhibited JAK kinases. Overall, IL-2 has complex actions on immune cells, and its role in immune responses is context-dependent. The amount of IL-2 produced, the expression of IL-2 receptors, and the presence of other cytokines in the cellular environment are critical in determining the biological outcome of IL-2 during an immune response. The development of new therapeutic approaches based on the administration of IL-2 or the use of antagonists of IL-2 production or signaling has the potential to treat a range of diseases. The modulation of IL-2 responses is context-dependent, and it is important to consider the specific needs of the disease when determining the appropriate therapeutic strategy.IL-2 is a pleiotropic cytokine that plays a central role in immune responses. It promotes CD8+ T cell and NK cell cytolytic activity, modulates T cell differentiation, and is essential for the development and maintenance of T regulatory (Treg) cells. IL-2 also mediates tolerance by promoting activation-induced cell death (AICD). This review discusses the molecular mechanisms and complex cellular actions of IL-2, its interactions with other cytokines, and its use in clinical medicine. IL-2 is a 15.5 kDa type 1 cytokine produced primarily by CD4+ T cells, and it binds to three classes of IL-2 receptors: IL-2Rα (CD25), IL-2Rβ (CD122), and IL-2Rγ (CD132). These receptors have different affinities for IL-2, with the high-affinity receptor being essential for signaling. IL-2 signaling is mediated through the Janus family tyrosine kinases JAK1 and JAK3, which activate STAT5, leading to the transcription of genes involved in T cell proliferation and differentiation. IL-2 also plays a critical role in T helper cell differentiation, including Th1, Th2, Th17, and Tfh cells. It promotes Th1 and Th2 cell differentiation while inhibiting Th17 cell differentiation. IL-2 is essential for the development of Treg cells and for maintaining immune homeostasis. In addition, IL-2 is involved in the regulation of autoimmune diseases and has been used as a therapeutic agent in clinical settings. However, its use is limited by its toxicity, including severe capillary leak syndrome. Recent advances in IL-2 therapy include the development of super-IL-2 variants that exhibit enhanced binding to IL-2Rβ and do not require IL-2Rα. These variants induce greater expansion of cytotoxic T cells but less expansion of Treg cells. In addition, inhibitors of IL-2 signaling, such as JAK inhibitors, have been developed as potential therapeutic agents. These inhibitors block the signaling pathways of IL-2 and other cytokines that utilize the inhibited JAK kinases. Overall, IL-2 has complex actions on immune cells, and its role in immune responses is context-dependent. The amount of IL-2 produced, the expression of IL-2 receptors, and the presence of other cytokines in the cellular environment are critical in determining the biological outcome of IL-2 during an immune response. The development of new therapeutic approaches based on the administration of IL-2 or the use of antagonists of IL-2 production or signaling has the potential to treat a range of diseases. The modulation of IL-2 responses is context-dependent, and it is important to consider the specific needs of the disease when determining the appropriate therapeutic strategy.