CD4 and breast cancer: CD4 Follicular helper T cell infiltration is associated with breast cancer prognosis,[27] and it can synergistically promote anti-tumor CD8 T cell response.[28] In addition, Follicular helper T cells also mediate the response of breast cancer mouse models to checkpoint inhibitors.[29] Regulatory T cells are an immunosuppressive subset of CD4 + T cells, and Treg cells maintain immune homeostasis by preventing excessive immune activation.