In terms of its anatomy and functions, the reproductive system of birds is significantly different from the one found in mammals. It consists of only the left ovary and the left oviduct, which is constantly exposed to ascending infections because of its connection with the cloaca. Hence, the proper functioning of humoral and cell-mediated local immune mechanisms in this system is very important for maintaining its physiological functions. The expression of Toll-like receptors and the presence of T and B lymphocytes have been demonstrated in both the ovary and the oviduct of chickens. CD4⁺ T cell subpopulation is distributed mainly in the lamina propria of the oviduct, whereas in the submucous membrane and muscular layer these cells are found less frequently. CD8⁺ T lymphocytes are equally distributed in all abovementioned layers of the oviduct wall. IgY⁺ B cells are distributed among the epithelial cells, and they are closely connected with the glandular tissue of the oviduct, mainly in the infundibulum, magnum, and uterus regions. IgA⁺ and IgM⁺ B cells are present in the entire oviduct, but mainly in the glandular tissue of the magnum. IgY⁺ B cells have not been detected in the ovary, unlike IgM⁺ B cells, which have been demonstrated in the ovary stroma. In addition to T and B cells, antigen-presenting cells are present in the follicle wall and in the oviduct. During the early stages of reproductive maturation, a decrease in the number of immunocompetent cells is observed in the reproductive system, and the local immnosuppression increases the susceptibility of birds to Salmonella Enteritidis infections. The number of T and B lymphocytes in the mucous membrane of the oviduct decreases with age, which facilitates infections of the reproductive system. Additionally, the local immune mechanisms of the reproductive system in birds involve the transfer of protective IgY, IgA and IgM maternal antibodies to hatching eggs. The local immune mechanisms of the reproductive tract are responsible for preventing infections that disturb the physiological functions of the reproductive system and for protecting eggs from contamination.
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