Extended Data Fig. 1: Robust peripheral B cell response to influenza virus vaccination.

a, ELISpot quantification of QIV-binding IgG-, IgM- and IgA-secreting PBs in blood 1 week after vaccination. Each symbol represents one participant (n = 8). b–e, Flow cytometry (b, c) and sorting (d, e) gating strategies for PBMCs (b, d) and FNA (c, e). Population counts per ml of blood and frequencies are presented in f and in Figs. 1d, 2f and Extended Data Fig. 2c, m. f, Kinetics of HA-binding PBs (CD20^lo HA⁺, open triangles) and activated B cells (ABCs) (CD20⁺ HA⁺, closed circles) in PBMCs, gated as in Fig. 1c, pre-gated IgD^lo CD19⁺ CD4⁻ live singlet lymphocytes as in b. Symbols at each time point represent one sample (n = 8). g, h, IgG plasma antibody titres against QIV and Tetanus/Diphtheria vaccine measured by ELISA (g) and haemagglutination inhibition titres against QIV-constituent viruses (h) before and 4 weeks after vaccination. Symbols at each time point represent one sample (n = 8). Horizontal lines in a, g and h represent means. Dotted lines represent limit of detection. P values were determined by paired two-sided Student’s t-tests.