A likely explanation for this is that the pertussis antigen is a whole cell preparation containing numerous ligands for pattern recognition receptors on macrophages and dendritic cells, leading to their secretion of IL-12 and IL-18, which is necessary to induce NK cells to secrete IFN-γ and become cytotoxic.4,13 Purified toxoids such as DT and TT lack such ligands and so, in vitro at least, NK cells can only be induced to respond in the presence of exogenous IL-12 and IL-18. This evidence concerns the gene IFNG and pertussis.