CD8A and viral infectious disease: At present, it is believed that effector CD8+ T lymphocytes can be mainly divided into Tc1, Tc2, Tc9, Tc17, follicular cytotoxic T (Tfc), follicular helper T (CD8+ Tfh) and regulatory T (CD8+ Treg) cells in response to different stimuli, such as tumors, viral infections, allergies, autoimmune diseases and transplantation (17, 18) (Table 1).