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 4 (IL-4) is a cytokine expressed by activated T cells, mast cells and bone marrow stromal cells. Originally, IL-4 was referred to as the B cell differentiation factor, B cell growth factor 1 or B cell stimulatory factor 1. The function of IL-4 is pleiotropic, i.e. stimulation, proliferation induction, and modulation of target cell functions. These target cells include B cells, but also T cells, NK and LAK cells, as well as antigen-processing cells like monocytes and macrophages.Synonyms: B-cell stimulatory factor 1, BSF1, Binetrakin, IL-4, Lymphocyte stimulatory factor 1, Pitrakinra