CD4 and viral infectious disease: In vivo: induce systemic immunity, such as lung RBD specific IgG antibody, and mucosal immune sIgA antibody, and activate Th1‐associated CD8+ and CD4+ T‐cells; faster removal of SARS‐CoV‐2 simulators from lungs; fight severe lung inflammation caused by live virus infection. In vitro: enhance the uptake of antigen ‐ presenting cells.