**5. Mucosal immune system**

Peyer's patches located at the level of the submucosa are sites of controlled uptake of antigens and activation of naive T and B lymphocytes. They are made up of several aggregates of B cells (lymphoid follicles) surrounded by rings of T cells, and also by antigen-presenting cells (APC), usually macrophages.

The epithelium that covers the dome of the lymphoid aggregates contains M cells, with the capacity to transport antigens from the intestinal lumen to the underlying lymphoid tissue, together with dendritic cells located in the lamina propria, whose formation is induced by various factors produced by the epithelial cells through stimulation of Toll-like receptors (TLR).

The products captured and processed by the APCs are presented to the naive T lymphocytes, initiating clonal expansion to collaborator or helper lymphocytes (Th1 or Th2) or regulatory T lymphocytes (Th3, Tr1, or CD25+/CD4+).

Following the process of antigenic stimulation, lymphocytes migrate into the mesenteric lymph nodes where further antigenic exposure and clonal expansion occur, then passing into the systemic circulation, returning to other mucosal surfaces, forming the so-called associated lymphoid tissue to the mucous membranes [4].

The humoral response occurs with the binding of the antigen to the IgM membrane of the B lymphocyte. The action signal generated by this binding stimulates a clonal expansion and as a consequence. The induction by antigen-specific T lymphocytes, or mediated by various factors, an additional differentiation takes place, which includes several processes of reorganization of the immunoglobulin chains, mainly of the IgA type. Activated B lymphocytes undergo a terminal differentiation process and become as plasma cells [5].

Secretory IgA (sIgA) is a powerful protector of the intestinal mucosa against bacterial invasions, constituting the first line of defense against multiple external infections including various viruses and other pathogens.
