Interleukin-2 (IL-2) is a powerful immunoregulatory lymphokine produced by T-cells in response to antigenic or mitogenic stimulation. It is expressed by CD4+ and CD8+ T cells, γδ T cells, B cells, dendritic cells, and eosinophils. IL-2/IL-2R signaling is required for T- cell proliferation and other fundamental functions which are essential for the immune response. The receptor for IL-2 consists of three subunits (55 kDa IL-2Rα, 75 kDa IL-2Rβ, and 64 kDa common gamma chain γc/IL-2Rγ) that are present on the cell surface in varying preformed complexes. Mature human IL-2 shares 56% and 66% amino acid sequence identity with mouse and rat IL-2, respectively. Human and mouse IL-2 exhibit cross-species activity.