Our knowledge about the protective value of the passive immunity provided by maternal IgG via placenta and SIgA via the milk is still incomplete. Although more detailed information is required it is clear that both forms of passive protection are important for the neonate. The immune response of neonates in secretions appears earlier and is more efficient than previously realized, providing SIgA as well as IgM in e. g. saliva. The presence of anti-antibodies (anti-idiotypes) in the transplacental IgG and milk SIgA may in fact actively prime the immune system of the fetus and the breast-fed infant. This could be one explanation why breast-fed infants seem to respond better to ordinary parenteral and peroral vaccines, in secretions as well as in serum than those fed a high or low protein formula.