CD4 and viral infectious disease: These include the ability of CD4+ T cells to differentiate into Th1 cells, which, in turn, have antiviral activities via the production of IFNγ [135,139]; T follicular helper (Tfh) cells, which help B cells in the development of neutralizing antibodies, and the long-term humoral immunity [140]; or CD4-CTL cells, which have direct cytotoxicity against several viral infections [141].