IL-23 (Interleukin-23) is a disulfide-linked cytokine composed of a p19 subunit that is unique to IL-23 and a p40 subunit that is shared with IL-12. It is produced by activated macrophages, microglia, and monocyte-derived dendritic cells in response to pathogens. IL-23 induces the earliest recruitment of neutrophils to the site of infection and promotes the development and maintenance of Th17 cells. It also enhances the development of Th17 mediated autoimmunity and tumor progression. IL-23 signals through a receptor complex consisting of IL-12 R beta 1 and IL-23 R. This complex is expressed in mouse Th1 and Th2 cells, bone marrow dendritic cells, activated macrophages and CD4+ CD45Rb(low) memory T cells.