Interleukin 23 (il-23) is a member of the iL12 cytokine family and composed of two subunits, il12p40 and il23p19. It is produced by antigen presenting cells and has been shown to promote the production and survival of a distinct lineage of T-cells called TH17 cells. A functional receptor for il-23 (the il-23 receptor) has been identified and is composed of Il-12Rβ1 and Il-23R. Il-23 is expressed chiefly by the macrophages and DCs. The il-23R is found on memory T cells, NKT cells, macrophages, DCs, and naive T cells upon activation by TGF-βand IL-6. The main biological effects of il-23 identified initially consist of stimulation of antigen presentation by DCs, T cell differentiation to Th17 cells, and production of interferon-γ (IFN-γ). Il-23 acts also as an end-stage effector cytokine through direct action on macrophages.