Interleukin 23 (IL-23) is a heterodimeric cytokine composed of the IL-12 p40 “soluble receptor” subunit and a novel cytokine-like subunit related to IL-12 p35, termed p19. Like IL-12, IL-23 binds to the IL-12R subunit IL-12Rβ1. However, it does not use IL-12Rβ2. IL-23 stimulates IFN-γ production and proliferation in PHA blast T cells, as well as in CD45RO (memory) T cells. Activated dendritic cells secrete detectable levels of IL-23. IL-23 promotes inflammatory responses such as upregulation of the matrix metalloprotease MMP9, and increases angiogenesis but reduces CD8 T-cell infiltration. Studies have indicated that IL-23 is an important molecular link between tumor-promoting pro-inflammatory processes and the failure of the adaptive immune surveillance to infiltrate tumors.