CD4 and influenza: Since these responses were directed against T-cell epitopes conserved within the H5 hemagglutinin proteins and since influenza-specific CD4- as well as CD8 T cells can contribute to protection against influenza infection [30], it can be speculated that the heterologous protection observed despite the presence of only low levels of neutralizing antibodies was, at least in part, due to the cross-reactive cell-mediated immunity.