Fetal blood natural killer cells in human pregnancy.

Flow cytometry was used to determine the number of circulating natural killer (NK) cells in 170 pregnancies undergoing cordocentesis for prenatal diagnosis at 17-38 weeks gestation and 16 undergoing cardiocentesis before social termination of pregnancy at 13-17 weeks. The number of NK cells decreased exponentially with gestation from a mean of 0.26 x 10(9)/l at 13 weeks to 0.09 x 10(9)/l at 38 weeks (r = -0.356, p < 0.001). When expressed as a percentage of the total number of lymphocytes, the proportion of NK cells decreased from a mean of 29% at 13 weeks to 6% at 38 weeks (r = 0.680, p < 0.0001). These data suggest that in early pregnancy the fetal immune response is dependent on innate rather than adaptive mechanisms.