indicating that they bound to sites other than the HA active site. Interestingly, antibodies of the latter type were predominant in the response (Fig. 3 A) . This specificity is reminiscent of antibodies against the recently discovered broadly neutralizing epitopes found on the HA stalk, rather than those located on the HA globular head that is more typical for neutralizing antibodies (Ekiert et al., 2009; Sui et al., 2009) . Importantly, five of these antibodies are indeed of similar specificity (including antibodies 70-5B03, 70-1F02, 1000-3D04, and a clonal pair from donor 1009: 3B05 and 3E06). These five antibodies bind with high affinity to most H1 strains including all from the vaccines of the past 10 yr, the 1918 pandemic strain, and to the H5 of a highly pathogenic number of patients, the frequent cross-reactivity and high number of somatic mutations support a model in which many of the plasmablasts induced by pandemic H1N1 infection arose from cross-reacting memory B cells.