Kinetics of vaccine-specific and bystander proliferative responses of T_mem cells following booster vaccination with TT. Six representative examples are shown. Vaccination (week 0) induces early (subjects 5, 3, and 10) and late (subjects 6, 8, and 9) TT-specific responses. Parallel kinetics were observed in the responses against 2 unrelated antigens (PPD and C albicans [C alb]) for which pre-existing immunity was evident, but no responses were detected against control antigens (Ctrl 1 and Ctrl 2) with no evident pre-existing immunity. Proliferative activity was measured by ³H-thymidine incorporation after 6 days of in vitro culture and reported for each time point as the mean value of stimulation index (SI) in triplicate cultures ± SD.