Human interleukin 2 (IL-2) is a 15-18 kDa glycoprotein of 133 amino acids after cleavage of a signal peptide of 20 amino acids. It contains three cysteine residues, two of which form a disulfide bond that is required for biological activity. It is a lymphokine synthesized and secreted primarily by T helper lymphocytes that have been activated by stimulation with certain mitogens or by interaction of the T cell receptor complex with antigen/MHC complexes on the surfaces of antigen-presenting cells. The response of T helper cells to activation is induction of the expression of IL-2 and receptors for IL-2 and, subsequently, clonal expansion of antigen-specific T cells. At this level IL-2 is an autocrine factor, driving the expansion of the antigen-specific cells. IL-2 also acts as a paracrine factor, influencing the activity of other cells, both within the immune system and outside of it. B cells and natural killer (NK) cells respond, when properly activated, to IL-2. The so-called lymphocyte activated killer, or LAK cells, appear to be derived from NK cells under the influence of IL-2. Murine IL-2 is approximately 63% identical to human IL-2, but contains a unique stretch of repeated glutamine residues. There is marked species cross-reactivity as human IL-2 has been found to be active on murine cell lines. Cells known to produce IL-2 include thymocytes, γ δ T cells, B cells, CD4+ and CD8+ T-cells, and neurons plus astrocytes.