Maternal immunoglobulin G (IgG), transferred across the placenta to the fetus during intrauterine life, is an important component of the neonatal immunological defence mechanisms against infection. There is controversy with respect to differences in placental transfer of the different IgG subclasses, and no definite data are available on a Japanese population. Therefore, we investigated placental transfer of IgG subclasses in a Japanese population. A total of 228 matched pairs of cord and maternal serum samples (20-42 weeks gestation) were assayed for each IgG subclass by an enzyme-linked immunosorbent assay. The mean values and hierarchy of cord/maternal concentration ratios of IgG subclasses at 40 weeks gestation were as follows: IgG1(1.47) > IgG3(1.17) = IgG4(1.15) > IgG2(0.80). The cord/maternal concentration ratios of all IgG subclasses were positively correlated to gestational age. The mean ratios for IgG1 and IgG4 nearly reached a plateau at 39 and 37 weeks gestation, respectively, while those for IgG2 and IgG3 increased until 41 weeks gestation. The ratios of all IgG subclasses for full-term deliveries were reciprocally correlated to the respective maternal IgG subclass serum levels. The results suggest that although all four IgG subclasses are actively transferred across the placenta, the efficiency of their transfer ranks in the order IgG1 > IgG3 = IgG4 > IgG2. The different results as to placental transfer of IgG subclasses in the literature might be due, at least in part, to different maternal IgG subclass serum levels in the populations studied.
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