T cells responding to low antigen concentrations are enriched in the memory compartments. (A) Dose–response curves of TT-, CMV-, and PPD-reactive T cell cultures from T_N, T_CM, and T_EM libraries of a single donor. Values on the left of each plot represent background proliferation in the absence of antigen. Shown are 10 representative cell lines for each library. (B) EC50 values for CMV-, TT-, and PPD-specific T cell cultures derived from different T cell subsets. Bars indicate median values. Data are representative of four independent experiments.