International Journal of Molecular Sciences Review Secretory IgA in Intestinal Mucosal Secretions as an Adaptive Barrier against Microbial Cells Bernadeta Pietrzak 1,* , Katarzyna Tomela 2 , Agnieszka Olejnik-Schmidt 1 , Andrzej Mackiewicz 2,3 and Marcin Schmidt 1,* 1 Department of Food Biotechnology and Microbiology, Poznan University of Life Sciences, 48 Wojska Polskiego, 60-627 Pozna´n,Poland;
[email protected] 2 Department of Cancer Immunology, Chair of Medical Biotechnology, Poznan University of Medical Sciences, 8 Rokietnicka Street, 60-806 Pozna´n,Poland;
[email protected] (K.T.);
[email protected] (A.M.) 3 Department of Diagnostics and Cancer Immunology, Greater Poland Cancer Centre, 15 Garbary Street, 61-866 Pozna´n,Poland * Correspondence:
[email protected] (B.P.);
[email protected] (M.S.) Received: 2 November 2020; Accepted: 2 December 2020; Published: 4 December 2020 Abstract: Secretory IgA (SIgA) is the dominant antibody class in mucosal secretions. The majority of plasma cells producing IgA are located within mucosal membranes lining the intestines. SIgA protects against the adhesion of pathogens and their penetration into the intestinal barrier. Moreover, SIgA regulates gut microbiota composition and provides intestinal homeostasis. In this review, we present mechanisms of SIgA generation: T cell-dependent and -independent; in different non-organized and organized lymphoid structures in intestinal lamina propria (i.e., Peyer’s patches and isolated lymphoid follicles). We also summarize recent advances in understanding of SIgA functions in intestinal mucosal secretions with focus on its role in regulating gut microbiota composition and generation of tolerogenic responses toward its members.