Interleukin 2 (IL-2), originally termed T cell growth factor (TCGF), was the first cytokine to be isolated, purified, and characterized at the molecular level. Structurally. IL-2 is prototypic of the interleukins. hematopoietic cytokines, and some more classic hormones, such as growth hormone and prolactin, in that it is a small globular glycoprotein, composed of four amphipathic antiparallel a helices. Functionally. IL-2 is somewhat atypical of the interleukins. in that its production is restricted to only antigen or mitogen-activated T lymphocytes (T cells), and its action is restricted to antigen-activated T cells and natural killer (NK) cells. IL-2 promotes the proliferation, differentiation, and survival of these target cells, such that it is obligatory for a physiologic antigen-specific acquired cellular immune response, and it also augments the innate host defenses through its effects on NK cells. IL-2 was discovered to mediate these biological activities by binding to IL-2-specific receptors, which are expressed transiently on antigen-activated T cells and continuously by NK cells.