The immune system reacts to a pathogen by activation of balanced network of the humoral and cellular immune responses. Subsequently the activated condition of the immune system will, after the elimination of the pathogen, be down-regulated to a balanced situation again. Control of the immune response requires efficient communication between the different cells involved in this response. This interaction is provided by cell/cell contact and by a complex array of mediators. Among these mediators cytokines, soluble factors produced by these cells, play an important role. Cytokines can act on other cells locally or distantly, but can be even auto regulating. Cytokines can behave stimulatory or inhibitory, or can even perform both activities, depending on the (pre)activation stage of the target cell. (3, 4) Interleukine 2 (IL-2) is a cytokine expressed by human lymphocytes that have been stimulated by antigens or mitogens. Without stimulation, IL-2 is produced in low amounts by TH1 cells in vivo. Originally, IL-2 was referred to as T cell growth factor. T cell hybridomas or a leukemia cell line such as Jurkat, produce increased amounts of IL-2 in vitro. Stimulation with IL-2 leads to rapid clonal expansion of T cells and IL-2 acts on a variety of cells in vivo, such as B cells, NK cells, monocytes and macrophages. Furthermore, IL-2 can induce activation of LAK cells.Synonyms: IL-2, T-cell growth factor, TCGF