Help T cells（Th）-17 cells are identified as a new subset of CD4 positive T cells unrelated to Th1 or Th-2 cells, and several cytokines are involved in regulating their activation and differentiation. Th-17 cells not only play an important role in host defense against extracellular pathogens, but also are associated with the development of autoimmunity and inflammatory response. Although the pathological mechanisms of periapaical lesions are not fully understood, accumulating evidence suggests that the abnormality of innate and adaptive immunity responses play an important role in periapical lesions inflammation. The identification of Th-17 cells can explain some of the anomalies seen in the Th-1, Th-2 axis and the immunopathogenesis of Th-17 cells in development of the periapical lesions. Periapical lesions, which form as a result of a root canal infection, are characterized by a host response to continuous antigenic stimulation in the infected canals and periapical bone destruction. Its pathological mechanisms are closely related to local inflammatory and immunity of periapical tissue. This review will introduce the Th-17 cells, the differentiation and fanctions of Th-17 cells in addition to the relationships between Th-17 cells and periapical lesions..
