Interleukin II (60-70) is a peptide with the sequence NH2-LEU-THR-PHE-LYS-PHE-TYR-MET-PRO-LYS-LYS-ALA-COOH and a molecular weight of 1373.7. Interleukin 2 (IL-2) is a cytokine signaling molecule that regulates leukocytes (often lymphocytes) responsible for immunity, and was the first interleukin molecule identified and characterized.

IL-2 is necessary for the growth, proliferation, and differentiation of T cells to become 'effector' T cells. IL-2 is normally produced by T cells during an immune response. It is necessary for the development of T cell immunologic memory, which depends upon the expansion of the number and function of antigen-selected T cell clones.IL-2 is also necessary for the maturation of a subset of T cells, termed regulatory T cells, during T cell development in the thymus.