Interleukin-12 (IL-12), is a cytokine that is secreted by activated phagocytes and dendritic cells and that induces interferon-γproduction by natural-killer and T lymphocytes. IL-12 is a 75 kDa heterodimer composed of a 35 kDa subunit (IL-12A p35) and a 40 kDa subunit (IL-12B p40) that is secreted by a wide variety of antigen presenting cells (APCs), including phagocytes, B cells and Langerhans cells. IL-12B has been found to be important for sustaining a sufficient number of memory/effector Th1 cells to mediate long-term protection to an intracellular pathogen. Overexpression of IL-12B was observed in the central nervous system of patients with multiple sclerosis (MS), suggesting a role of this cytokine in the pathogenesis of the disease. The promoter polymorphism of IL-12B has been reported to be associated with the severity of atopic and non-atopic asthma in children.