Carstens et al. [36] performed an immunohistochemical study and found that the tumor immune microenvironment of pancreatic ductal adenocarcinoma is predominantly constituted by CD8 positive cytotoxic T lymphocytes and CD4 positive helper T lymphocytes, proving that they are an independent indicator of positive prognosis. Here, CD4 is linked to pancreatic ductal adenocarcinoma.