Interleukin 12 (IL-12p70) is a heterodimeric cytokine composed of p35 and p40 subunits. IL-12 is produced by monocytes, macrophages, dendritic cells, neutrophils and B cells in response to bacterial products and cytokines such as IFN-γ. The IL-12 receptor is expressed on T, NK, and dendritic cells. Upon binding, IL-12 initiates signaling via the JAK/STAT signaling pathway and stimulates NK, B, and T cells to produce IFN-γ (Watford et al.). It also regulates cytokine synthesis, proliferation of T and NK cells, and stimulates differentiation of CD4+ and CD8+ T cells (Germann and Rüde). Mice that are deficient in IL-12 are susceptible to many intracellular pathogens and have impaired IFN-γ secretion, Th1 differentiation and NK cytolytic activity; however, Th2 development and IL-4 production are enhanced (Watford et al.).