These data are fully consistent with a scenario in which a whole cell antigen such as pertussis contains ligands for Toll-like receptors16 and so induces accessory cells to secrete cytokines such as IL-12 and IL-18, whereas purified proteins such as TT and DT do not; exogenous LCC induces expression of CD25 (and so the high-affinity IL-2R) on NK cells allowing them to respond to IL-2 from vaccine-specific CD4+ T cells. Here, CD4 is linked to pertussis.