Naïve CD8+ T cells circulate through the body, until their cognate antigen is presented to them by an antigen-presenting cell, for example a dendritic cell, in a secondary lymphoid organ, for example a lymph node. T cells then become activated, proliferate, differentiate, and migrate to the tumor. Upon entering the TME, they can mediate an effective anti-tumor response through direct cytolytic activity, mediated by perforin and granzymes, and the secretion of cytokines like IFNγ and TNFα. IFNγ, interferon γ. MHC-I, major histocompatibility complex I. TCR, T cell receptor. TME, tumor microenvironment. TNFα, tumor necrosis factor α.