Learning immunology from the yellow fever vaccine: innate immunity to systems vaccinology.

Despite their great success, we understand little about how effective vaccines stimulate protective immune responses. Two recent developments promise to yield such understanding: the appreciation of the crucial role of the innate immune system in sensing microorganisms and tuning immune responses, and advances in systems biology. Here I review how these developments are yielding insights into the mechanism of action of the yellow fever vaccine, one of the most successful vaccines ever developed, and the broader implications for vaccinology.