Several proteins, such as polymeric immunoglobulin (Ig) receptor/secretory component (pIgR/SC), immunoglobulins (Igs) and joining (J) chain, and cellular components of the secretory immune system (SIS) of the human embryo and fetus were analyzed and compared with reviewed information concerning SIS organization and function. All organs and structures, including extracorporeal 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 during its entire development. SC, Igs and J chain were already widely distributed in 4-week-old embryos and were later seen in the mucosa and glands of the digestive, respiratory and urogenital tracts, ovaries, testis, endocrine glands, extracorporeal tissues, blood-brain and other blood-organ barrier structures, as well as in serous membranes. The presence of protein transport and later of cellular components suggests an active role for SIS not only in mucous membranes, but also in blood-tissue barriers. Loss of morphological contact between epithelial structures and mucous membranes during organogenesis of some endocrine organs (hypophysis, pancreatic islets, etc.) is followed by the disappearance of SC as a result of cessation of Ig exocytosis. Secretion of Igs increased in the epithelium and glands of the digestive and respiratory tracts following massive antigenic attack in cases of acute chorioamnionitis. All of this demonstrates that SIS is a widely branching immune system which appears and acts early in the human embryo, before the lymphoid system is formed, using IgG and IgA of maternal origin. IgA and IgM-synthesized lymphocytes appear in 9-week-old fetuses.