<qa>  What is the cause of immunoglobulin G4 positive plasma cells?Investigators have previously found that the infiltration of immunoglobulin G4IgG4 positive plasma cells in tumor tissue predicts a poor prognosis of pancreatic cancer after curative resection. Investigators further attempt to explore the possible roles of IgG4 and the inducer of IgG4, interleukin-10IL-10, in the chemotherapy of pancreatic cancer. In this primary study, investigators plan to observe the dynamic changes of sera IgG4 and IL-10 in peripheral blood after gemcitabine-based chemotherapy and analyze the correlations of IgG4 and IL-10 with the response of gemcitabine and overall survival of pancreatic cancer. 