CD4 and viral infectious disease: Unlike foreign protein antigens that may pass through a cross-presentation pathway with limited efficiency, LNP-mRNA vaccines elicit the translation of proteins in the cytoplasm, which can then be processed into peptides and presented to major histocompatibility complexes type I (MHC-I) in the same way as heterologous antigens produced by viral infection, resulting in the innate ability to stimulate strong humoral and cellular immunity, among which CD4+ T-cells and CD8+ T-cells are important detection indicators.