CD8A and cancer: Once activated, CD4+ T cells (also called T helper (Th) cells) differentiate into heterogeneous subsets (e.g., Th1, Th2, regulatory T (Treg) cells, and follicular helper T (Tfh) cells), while CD8+ T cells (also called cytotoxic T lymphocytes (CTLs)) can kill cancer cells through secreting cytokines (e.g., TNF-α and INF-γ) or direct action [49].