During the last three decades, the development and commercialization of conjugate vaccines against Haemophilus influenzae type b (Hib), pneumococcus, and serogroups C, A, W, and Y of meningococcus contributed to the virtual elimination of bacterial meningitis caused by the bacteria included in the vaccines and to the prevention of diseases that used to cause more than a million deaths annually (1, 2). Despite the great impact on public health of these vaccines, our understanding of the way these vaccines work is still limited, and we have many unanswered questions. In PNAS, Sun et al.(3) report new mechanistic insights on conjugate vaccines.