IL-12 is a heterodimeric pro-inflammatory cytokine composed of two subunits encoded by different genes, IL-12A (p35) and IL-12B (p40), which by themselves have no activity. IL-12 is a strong inflammatory mediator produced by activated macrophages, monocytes and dendritic cells and appears to be important in T cell maturation and NK cell activity. While heterodimeric IL-12 is detected in low amounts, the p40 subunit is produced in excess. This may be because the p40 subunit also binds to p19 to form IL-23, which acts on CD4+ Th1 effector cells and plays an important role in later stages of infection. Additionally, p40 may form an IL-12 p80 homodimer which acts as an antagonist for IL-12 and IL-23 by competing for their receptors.