IL-2 (Interleukin-2) is a critical cytokine in T cell biology. It promotes T cell activation and expansion, the development, maintenance and function of regulatory T cells (Treg), and the differentiation of CD8+ T cells into terminal effector cells and memory cells. IL-2 signals through a receptor complex composed of CD25/IL-2 R alpha, IL-2 R beta, and the Common gamma Chain (gamma c). IL-2 R beta is also a component of the IL-15 receptor complex. Gamma c is also a signaling subunit in the receptors for IL-4, -7, -9, -15, and -21.