<qa>  What is the most common clinical manifestation of IgG4-related sclerosingHuman immunoglobulin GIgG is a family consisting of four members, IgG1, IgG2, IgG3 and IgG4.IgG4 is regarded as an inhibitory IgG which can inhibit the activation of immune responses1. Recently it was reported that IgG4 could weaken the activation of macrophages to promote cancer progression2. Autoimmune pancreatitisAIP is the most common clinical manifestation of IgG4-related sclerosing diseasesIRSD which is characterized by abundant infiltration of IgG4 positive plasma cells3.Although there are abundant infiltrations of IgG4 positive plasma cells in pancreatic lesion of AIP, the correlation of IgG4 positive plasma cells with pancreatic cancer has never been reported.Investigators have previously found that higher level of infiltration of IgG4 positive plasma cells in tumor tissue predicts a poor prognosis of pancreatic cancer after curative resectionnot published.Investigators further attempt to explore the possible roles of IgG4 and the inducer of IgG4, IL-10, in the chemotherapy of pancreatic cancer.Since gemcitabine is the first line chemotherapeutic drug for 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. 