Purified B cells were costimulated with CpG<sub>2006</sub>, A/Victoria/361/2011 viruses and IL-15. Stalk-reactive IgG in supernatants was detected at day 6. Chimeric molecules cH5/3, cH4/7 and cH5/1 were used for assessing antibodies against H3 stalk, H7 stalk and H1 stalk, respectively. Each symbol and line represents an individual donor. (A) H3 stalk-reactive antibodies. (B) H7 Stalk-reactive antibodies. (C) H1 stalk-reactive antibodies
