Plasmablasts from patients infected with pandemic H1N1 influenza were highly cross-reactive to prepandemic influenza strains As the plasmablasts are specifically induced by the ongoing immune response, we can learn about the origin of the B cells The mAb column indicates whether mAbs were made from the plasmablasts of these patients. (Table I) , mutations had accumulated at a significantly lower frequency than the IgG controls ( Fig. S3 A; P < 0.0001), suggesting a unique circumstance such as a low-level or lacking primary response. Detailed sequence characteristics for pandemic H1N1-induced plasmablasts are provided in Tables S1-S3. Though based on a limited or vaccine response. The ready detection of clonal expansions at a mean frequency of 16.5% of the cells for the six patients supports this view (based on CDR3 sequence similarity; Fig. 2 C). Since the discovery of somatic mutation, it has been appreciated that mutations progressively accumulate on variable genes after repeated immunizations (McKean et al., 1984) . Thus, we can gain insight into the origin of the pandemic H1N1 response by comparing the somatic mutation frequency of the plasmablasts present during H1N1 infection to that of other plasmablast responses. The PCR strategy allowed isolation of either IgG or IgA transcripts and identified 68% IgG and 32% IgA plasmablasts from the patients. Similar to plasmablasts induced by annual vaccination (Wrammert et al., 2008) , or after a fourth booster vaccine to anthrax, the variable genes of novel H1N1-induced cells from five of the six patients harbored high numbers of somatic mutations