The weakened immune system of tumor patients cannot produce effective immune response to tumor cells, thus allowing tumor cells to escape from the body’s immune surveillance is one of the important mechanisms of tumor development and metastasis. The team of Professor Yu Xian-F of the Department of Pancreatic Surgery, Fudan University Pancreatic Tumor Institute/Fudan University Cancer Hospital has long focused on the role of immune microenvironment in the development of pancreatic cancer, and found in a previous study that the immune status of pancreatic cancer patients can directly affect the long-term survival after surgery. The specific immune function of the body includes humoral immunity and cellular immunity, among which cellular immunity mediated by various T cells is the main body of anti-tumor immunity, T cells are important immunoreactive cells, which can be divided into CD4+ T cells and CD8+ T cells according to the surface CD molecules, among which CD4+ T cells play an important immunoregulatory role in the overall cellular immune response. Based on their functions, CD4+ T cells are classically divided into three subpopulations: Th1, Th2 helper cells, and T regulatory (Treg) cells. th1 and Th2 helper cells play an important role in anti-infection and anti-tumor responses by secreting cytokines such as IFN-γ and IL-12. Th2 cells promote humoral immune response by secreting cytokines such as IL-4, IL-5 and IL-13, and on the other hand are involved in the pathogenesis of asthma and systemic autoimmune diseases. Treg cells, on the other hand, play an important role in maintaining the immune homeostasis of the body. The functional classification of T cells described above has enhanced the understanding of the immune regulatory network and also provided a basis for intervention in diseases. However, as research progressed, it was found that the above T-cell functional classification could not fully explain the pathogenesis of some diseases such as autoimmune diseases, infections and allergic reactions. In 2005, a subpopulation of cells different from Th1, Th2 and Treg, which do not express IL-4 or IFN-γ but secrete high levels of IL-17, was named Th17 cells, which were shown to induce severe autoimmune reactions by secreting the inflammatory mediator IL-17. The absence of Th17 cells prevents or attenuates the development of autoimmune diseases such as autoimmune encephalomyelitis (EAE). More importantly, it was found that there is a subtle regulatory relationship between Th17 cells and the induction of differentiation process of Th1 cells and Treg. It was found that Th17 cells play an important role in anti-infection immunity, autoimmunity and tumorigenesis development. Although Th17 has been shown to be associated with many tumors such as lung, breast, ovarian, hepatocellular liver, and pancreatic cancers, however, Th17 exhibits different roles in the development of different tumors. Studying the function and differentiation mechanism of Th17 cells and the relationship with other T-cell subsets is of great importance for gaining insight into the regulation of body immunity, understanding disease pathogenesis, and seeking more effective targets for disease intervention.