Attributing the observed effects of the DTwP / MV interactions entirely to shifts in the Th1 / Th2 profile is likely to be an over simplification, but provides a good starting point for unravelling this phenomenon. Other arms of the immune response that need to be considered are regulatory T cells (Tregs) and the Th17 inflammatory T cell lineage. The former are a heterogeneous group of naturally occurring and induced T cells, and have been shown to play a regulatory role in immunity to a number of infectious diseases, but almost nothing is known about their generation or functional role in vaccine immunogenicity in humans. Newborns are known to have high levels of functional Tregs, and these are likely to be essential in controlling the immune response to antigens encountered in early life, as well as modulating self reactive responses. Measles vaccination causes suppression of T cell responses, and measles vaccinated children upregulate FOXP3 expression (Ota et al, personal communication), suggesting a role for Tregs. Thus Treg induction is likely to be an important component of the immune response to MV.