In this study, we compared the TIL counts and immune system-related characteristics such as levels of CD4, CD8, Foxp3, PD-L1, PD-L2, and HLA class I antigen between primary breast tumors and the corresponding brain metastases via immunohistochemistry to better characterize the immune microenvironment of brain metastases. This evidence concerns the gene CD4 and breast neoplasm.