CD4 and viral infectious disease: The MHC class II alleles (HLA-DR, -DQ and -DP) present epitopes to CD4+ T cells [epi-HLA class II → CD4+ T-cells] that in turn elicit adaptive immune responses against viral infections by facilitating the induction of CD8+ cytotoxic T-lymphocytes (CTLs), production of cytokines crucial for survival, and maturation of B cells [10,11,12,13].