CD4 and influenza: The response of PBMC from the same donor to N1 peptides was positive, whereas the response to H5 peptides was at background level (Figure 2, panel E and D: 1.49% vs. 0.14% of IFN-γ+ CD4+ T cells), a finding that suggests that N1 seems to be the main target for cell-mediated cross-type immunity against influenza (H5N1) and influenza (H3N2)/(H1N1) vaccine strains.