Interleukin 23 (IL-23) is a member of the IL-12 cytokine family and composed of two subunits, IL-12 p40 and IL-23 p19. 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-β and 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. This kit can be used to determine relative mass values for IL-23 p40.