Among PBMC stimulated with vaccine antigen alone (i.e. without LCC) there is highly significant up-regulation of both CD25 and IFN-γ by NK cells in response to pertussis (median 1·3%, range 0·0–4·6), a lesser (but still significant) response to DT (median 0·1%, range 0·0–1·3) and no significant response to TT (median 0·1%, range 0·0–1·3) (Fig. 1b). This evidence concerns the gene IL2RA and pertussis.