Detection of tetanus toxoid specific memory B cells. (A) Gating scheme for detection of TTCF tetramer-positive B cells among class-switched memory B cells. (B) Identification of TTCF tetramer-positive class-switched memory B cells from 4 donors. The elapsed time since the last tetanus boost is indicated (top). Labeling with a control tetramer (membrane-proximal domain of CD80) is shown for each donor (bottom). Frequencies represent tetramer-positive cells among CD19⁺ CD27⁺ IgM⁻ B cells. (C) Scatter plot of tetramer⁺ cells detected per 1 × 10⁶ B cells, with the mean indicated by a solid vertical line. The mean frequency was 52.0 cells per 1 × 10⁶ B cells with the TTCF tetramer, and 4.3 cells per 1 × 10⁶ B cells with the control tetramer. (D) Scatter plot showing percentage of tetramer-positive cells within the memory B-cell population. The mean frequency of TTCF tetramer-positive was 0.0447% (0.0032% for the control tetramer).