We have shown, using the individual components of the diphtheria toxoid/tetanus toxoid/whole-cell pertussis vaccine, that activation of NK cells after restimulation with vaccine Ags is heterogeneous, with CD56bright and CD56dimCD57− NK cells being most responsive as measured by surface expression of the high-affinity IL-2 receptor (CD25) and accumulation of intracellular IFN-γ (CD25+IFN-γ+) (6). This evidence concerns the gene IFNG and tetanus.