IL-23 protein is a heterodimer, sharing a p40 subunit with IL-12, but having a distinct p19 subunit, which is IL-23 specific. Prior to the discovery of IL-23, IL-12 had been proposed to represent a key mediator of inflammation in mouse models of inflammation.However, many studies aimed at assessing the role of IL-12 had blocked the activity of IL-12p40, and were therefore not as specific as thought. Studies which blocked the function of IL-12p35 did not produce the same results as those targeting IL-12p40 as would have been expected if both subunits formed part of IL-12 only.
IL-23 was shown to be involved in inflammation of the nervous system and autoimmune diseases.IL-23 was shown to facilitate development of inflammation in numerous other models of immune pathology where IL-12 had previously been implicated including models of arthritis, intestinal inflammation, and psoriasis.