CD4 and influenza: Previously studies have shown that there is cross-reactive antibody and CD4+ T cell responses between influenza B and A HAs in both humans and mice [32], [33], [34] and there is an estimated 23–27% amino acid homology between influenza B and A HA proteins [34], [35], [36]; which may be a reason for a strong IgG response towards influenza B. Also, in close proximity to the fusion peptide in the stalk of the HA, a highly conserved B cell epitope have been identified and shown to be cross-reactive within influenza A subtypes and influenza B HA [37].