Re-immunization of individuals with persistent GCs engages affinity-matured PBs. Related to Fig. 6. (A) Kinetics of HA-binding PBs (CD20^lo HA⁺) in blood from three participants (04, 05, 11) by flow cytometry. Cells were pre-gated on CD4⁻ CD19⁺ IgD⁻ live singlets. (B) Optical density (OD) at 490 nm at 15 µg/ml of 2019 QIV–binding clonally unique mAbs generated from singly sorted PBs from week 1 after vaccination. Positive binding is defined as greater than two times the OD 490 value for antibody binding to BSA. (C and D) Flow cytometry gating of total GC B cells (CD20⁺ CD38^int) and HA-binding GC B cells (CD20⁺ CD38^int HA⁺) in the lymph node (LN) from participant 05 (C) and participant 04 (D). Cells were pre-gated on CD4⁻ CD19⁺ IgD⁻ live singlets. (E) OD at 490 nm of 15 µg/ml of 2019 QIV–binding clonally unique mAbs generated from GC B cells at the indicated time points after vaccination. Positive binding defined as greater than two times the OD 490 value for antibody binding to BSA. (F) Unsupervised clustering visualized via UMAP of B cells from blood, LN, and bone marrow (BM) scRNA-seq samples in participant 04. Each dot represents a cell, colored by phenotype as defined by transcriptomic profiles. Naïve B cells (gold), PBs (red), ABCs (green), GC B cells (blue), LNPC (red), RMBs (lavender), and plasma cells (PC, red) populations are pooled from all time points (first panel). QIV-specific cells at each week after vaccination are colored as described. N.D., no data.