Role of T cells in infection, autoimmunity, and cancer. (A) During infection, CD8⁺ T cells directly recognize and eliminate pathogenic peptides presented on the surfaces of infected cells. (B) T cells do not react against self-antigens owing to central and peripheral immunogenic tolerance. Failure of the checkpoints described in panels A and B can cause uncontrolled expansion of self-reactive T cells, which leads to the development of AI diseases. (C) In steady-state, T cells survey, patrol, identify, and destroy abnormal cells, including cancer cells. pDC, plasmacytoid DC.