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STUDY QUESTION: How does the placenta protect the fetus from immune rejection by the mother? SUMMARY ANSWER: The placenta can produce IgG that is glycosylated at one of its Fab arms (asymmetric IgG; aIgG) which can interact with other antibodies and certain leukocytes to affect local immune reactions at the junction between the two genetically distinct entities. WHAT IS KNOWN ALREADY: The placenta can protect the semi-allogenic fetus from immune rejection by the immune potent mother. aIgG in serum is increased during pregnancy and returns to the normal range after giving birth. aIgG can react to antigens to form immune complexes which do not cause a subsequent immune effector reaction, including fixing complements, inducing cytotoxicity and phagocytosis, and therefore has been called 'blocking antibody'. STUDY DESIGN, SIZE, DURATION: Eighty-eight human placentas, four trophoblast cell lines (TEV-1, JAR, JEG and BeWo), primary culture of human placental trophoblasts and a gene knock-out mouse model were investigated in this study. PARTICIPANTS/MATERIALS, SETTING, METHODS: The general approach included the techniques of cell culture, immunohistochemistry, in situ hybridization, immuno-electron microscopy, western blot, quantitative PCR, protein isolation, glycosylation analysis, enzyme digestion, gene sequencing, mass spectrophotometry, laser-guided microdissection, enzyme-linked immunosorbent assay, pulse chase assay, double and multiple staining to analyze protein and DNA and RNA analysis at the cellular and molecular levels. MAIN RESULTS AND THE ROLE OF CHANCE: Three major discoveries were made: (i) placental trophoblasts and endothelial cells are capable of producing IgG, a significant portion of which is aberrantly glycosylated at one of its Fab arms to form aIgG; (ii) the asymmetrically glycosylated IgG produced by trophoblasts and endothelial cells can react to immunoglobulin molecules of human, rat, mouse, goat and rabbit at the Fc portion; (iii) asymmetrically glycosylated IgG can react to certain leukocytes in the membrane and cytoplasm, while symmetric IgG from the placenta does not have this property. LIMITATIONS, REASONS FOR CAUTION: Most of the experiments were performed in vitro. The proposed mechanism calls for verification in normal and abnormal pregnancy. WIDER IMPLICATIONS OF THE FINDINGS: This study identified a number of new phenomena suggesting that aIgG produced by the placenta would be able to react to detrimental antibodies and leukocytes and interfere with their immune reactions against the placenta and the fetus. This opens a new dimension for further studies on pregnancy physiology and immunology. Should the mechanism proposed here be confirmed, it will have a direct impact on our understanding of the physiology and pathology of human reproduction and offer new possibilities for the treatment of many diseases including spontaneous abortion, infertility and pre-eclampsia. It also sheds light on the mechanism of immune evasion in general including that of cancer. STUDY FUNDING/COMPETING INTERESTS: Funded by the Li KaShing Foundation and the National Natural Science Foundation to Jiang Gu, grant no. 30971150 and 81030033. There is no competing interest. TRIAL REGISTRATION NUMBER: N/A. [ABSTRACT FROM AUTHOR]