Interleukin-2 (IL-2) is a cytokine that plays a crucial role in the immune system. It is primarily produced by activated T cells and has multiple functions. IL-2 promotes the proliferation and differentiation of T cells, enhancing their ability to fight infections and tumors. It also stimulates the growth and activity of natural killer (NK) cells, which are important for immune defense. Additionally, IL-2 can regulate the activity of regulatory T cells (Tregs), helping to maintain immune tolerance and prevent autoimmune diseases. In clinical settings, IL-2 has been used to treat certain cancers by boosting the immune response, although its use is limited due to potential side effects. Its discovery and research have significantly advanced our understanding of the immune system and provided new strategies for immunotherapy.