Detection of tetanus toxoid specific plasmablasts after vaccination. (A) Plasmablasts were identified on the basis of intermediate CD19 and high CD27 expression. Fluorescence-activated cell sorter plots were gated on total CD19⁺ B cells, which were negative for a panel of exclusion markers (CD3, CD14, CD16, and 7AAD). Numbers adjacent to the gate represent the percentage of plasmablasts within the CD19⁺ B-cell population. (B) Identification of TTCF tetramer-positive cells within the plasmablast population (gated in panel A). Numbers adjacent to gate represent the percentage of TTCF tetramer-positive within the plasmablast population (identified in panel A). (C) Frequency of tetramer-positive plasmablasts within the total CD19⁺ B-cell population during the first 2 weeks after vaccination. (D) Variable gene segment usage by unique B-cell clones for heavy (n = 26) and light (n = 25) chains from single cell sorted tetramer-positive plasmablasts isolated from days 6 and 7. (E) Scatter plot of somatic mutations detected in variable gene segments of heavy and light chains with mean values (V_H = 25.3 and V_K = 16.2) indicated by vertical solid lines.