The balancing square model. The 4 T-cell types (Th1, Th2, Th17, and Treg) antagonize each other. The Th1-promoting cytokine, IL-12, is inhibitory to Th2-cell development, whereas the Th2-promoting cytokine, IL-4, blocks Th1-cell development. The Th1-derived IFN-γ blocks Th17-cell development. The Treg inhibits the development of both Th1 and Th2 cells by direct contact.