Interleukin-2 (IL-2) is a 133 amino acid glycoprotein with one intramolecular disulfide bond and variable glycosylation. It is secreted by activated T cells and induces proliferation and maturation of activated T cells, natural killer cells, and lymphokine activated killer cells. IL-2 also stimulates proliferation of antibody-producing B cells, activates neutrophils, and induces mononuclear cells to secrete IFN-γ and TNF-α and -β. Moreover, studies have shown that IL-2 is required for activation-induced apoptosis, an important hemeostatic mechanism in the immune system, which is involved in the maintenance of peripheral tolerance to self-antigens.