IL12B is a subunit of interleukin 12, a cytokinethat acts on T and natural killer cells, and has a broad array ofbiological activities. Interleukin 12 is a disulfide-linkedheterodimer composed of the 40 kD cytokine receptor like subunitencoded by this gene, and a 35 kD subunit encoded by IL12A. Thiscytokine is expressed by activated macrophages that serve as anessential inducer of Th1 cells development. This cytokine has beenfound to be important for sustaining a sufficient number ofmemory/effector Th1 cells to mediate long-term protection to anintracellular pathogen. Overexpression of this gene was observed inthe central nervous system of patients with multiple sclerosis(MS), suggesting a role of this cytokine in the pathogenesis of thedisease. The promoter polymorphism of this gene has been reportedto be associated with the severity of atopic and non-atopic asthmain children.