There are many reports on the association between infiltrating immune cells andcancer prognosis. It is generally thought that cancer cells escape from the immune surveillance system in vivo. Cells associatedwith tumor immunosuppressive mechanisms include macrophages, regulatory T cells, bone marrow-derived immunosuppressive cells(MDSC), andneutrophils. These immunosuppressive cells enhance the production of TGF-b andIL -10 andthe expression of PDL-1 by cytokines producedby stromal cells such as cancer cells andfibroblasts, thereby inducing cytotoxic T cells lymphocytes(CTL). On the other hand, it has been proved that CD8+ T cells react in an antigen-specific manner even in advanced gastric cancer, suggesting the possibility that memory T cells, NK cells andNKT cells in gastric cancer tissues correlate with goodprognosis. Recently, it has been reportedthat the presence of follicular lymphoidstructure calledtertiary lymphoidstructure(TLS)in gastric cancer tissue is associatedwith favorable prognosis. Although immune responses are suppressedin gastric cancer tissues, the effectiveness of an immune checkpoint inhibitor(anti-PD-1 antibody)has been provedin 2017. The tumor-infiltrating immune cells is known as a predictive effect biomarker. As cancer genome research progresses, which type of immune response is induced is gradually being elucidated in near future. Thus, assessing the invasive morphology and function of various tumorinfiltrating immune cells is considered to be extremely important in Precision Medicine for gastric cancer.