Our study aimed to investigate TILs in the TME of HER2(+) GC during NACT and the effect of additional trastuzumab in preoperative chemotherapy and changes in CD8+ T cells, FoxP3+ T cells, and TLS in the TME of HER2(+) GC patients after preoperative chemotherapy and trastuzumab treatment by using the mIHC technique. The gene discussed is CD8A; the disease is gastric cancer.