CD4+ T helper cells regulate appropriate cellular and humoral immune responses to a wide range of pathogens and get involved in many diseases progress. The balance of the earliest determined CD4+ T helper cell subsets, Th1 and Th2, play an important role in allergy and autoimmune diseases. During the research, Animal models in immunology research are necessary and always the powerful tools for the basic scientific research. With the new sequence technologies, the finding of key gene mutation in Th1/Th2 cells has been proved to be related to human diseases. Here, we review four animal models about four key genes in Th1/Th2 cells to introduce the balance between Th1/Th2 cells. Furthermore, the related genetic mutations in human diseases and the new therapies are reviewed in this chapter, which show the importance of Th1/Th2 cells in human diseases further.