CD4 and influenza: Given that Tfh cells have been shown to be the limiting cell subset in the GC reaction [51–53], as well as Tfh cell magnitude correlating with formation of broadly neutralizing antibodies [43–46], we proposed that by directly manipulating Tfh cell magnitude in mice via heterologous infection or immunization priming of CD4+ T cells we would enhance the germinal center reaction and its products compared to primary influenza infection alone.