Placental transfer favours high avidity IgG antibodies.

The aim of the present study was to evaluate if maternal-foetal antibody placental transfer may be affected by antibody avidity. We compared the avidity index (AI) of IgG antibodies to tetanus toxoid (TT) and to type 3 pneumococcal antigen (Pn) in cord blood of 10 healthy term and 8 preterm infants and in their mothers' sera at delivery. In order to evaluate whether a heavier antigenic exposure may influence the placental transfer, we also studied 15 Pakistani maternal sera and cord blood pairs. TT- and Pn-specific antibody AI was significantly higher in Italian and Pakistani term infants than in their mothers, while a significant difference in specific TT antibody AI, but not in specific Pn antibody AI was observed between preterm infants and their mothers. Italian and Pakistani cord blood/maternal serum pairs showed comparable values of AI. Our data suggest that high avidity antibodies preferentially cross the placenta; this seems to start early during gestation and appears to be related to the nature of the antigen to which the antibodies are directed, but not to the degree of antigenic exposure.