The role of joining (J) chain, one of the protein components of the secretory immune system (SIS), in the immune reactions of the human embryo and fetus was analyzed on the basis of data from the literature and our previous studies. All organs and structures, including extra-corporeal ones, of 18 embryos (4-8 weeks of development) and 45 fetuses (9-38 weeks) were studied using methods of pathomorphology, immunohistochemistry and morphometry. This approach enabled us to analyze the problem in the whole organism throughout its embryonic and fetal development. J chain, as well as polymeric immunoglobulin (Ig) receptor-secretory component (pIgR/SC) and Igs, are already widely distributed in 4-week-old embryos before the appearance of the common immune system. The whole complex of protein components of the SIS was seen in mucous layers, and in blood-tissue and tissue-tissue barrier structures. Therefore, we can consider two parts of the SIS: mucosal and barrier. Already in embryos, an increase in the functional activity of the SIS following massive antigenic attack in cases of acute chorioamnionitis reflects the increased exocrine secretion of Igs. The J chain appears to participate in the endocytosis but not exocytosis of Igs. J chain and Igs, but not pIgR/SC, were present in cells of the heart, endocrine glands, gonads and some other organs. The exocrine secretion of Igs, the main function of the SIS, is absent in these organs, and, they are therefore, not considered part of the SIS.

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