Because the
lumen of the
gastrointestinal tract is exposed to the external environment, much of it is populated with potentially
pathogenic
microorganisms. In order to facilitate proper immune recognition and response to potential invaders, Peyer’s patches function as primary inductive sites for antibody production in the gut, specifically IgA. Germinal centers within the patches contain IgA+ B-cells, which differentiate into antibody producing plasma cells, supporting antigen specific and high affinity IgA responses. Peyer's patches thus establish their importance in the immune surveillance of the intestinal lumen and in facilitating production of the immune response within the
mucosa. Pathogenic microorganisms and other
antigens entering the intestinal tract encounter
macrophages,
dendritic cells,
B-lymphocytes, and
T-lymphocytes found in Peyer's patches and other sites of
gut-associated lymphoid tissue (GALT). Mechanistically, activated B-cells in Peyer’s patches form active germinal centers, supported by CD4 T cells, promoting IgA production. Subsequently, activated B-cells migrate to the subepithelial dome, where they interact with antigen-presenting dendritic cells. These interactions promote transforming growth factor beta (TGF-β) signaling, which is required for class switching recombination from IgM to IgA and the generation of IgA committed B-cells that subsequently differentiate into plasma cells. Peyer's patches thus act for the gastrointestinal system much as the
tonsils act for the
respiratory system, trapping foreign particles, surveilling them, and destroying them. Despite these immune functions, Peyer's patches are distinct from other secondary lymphoid organs, as mice treated with
CD122-targeted
interleukin-2 (IL-2) complexes, which are normally associated with activation of immune cell populations, displayed reductions in Peyer's patch cellularity, specifically increased B-cell apoptosis. Although B-cell populations recovered after treatment ended, these findings may provide insight into the role that Peyer's patches may play in gastrointestinal diseases. Structurally, Peyer's patches are covered by a special follicle-associated epithelium that contains specialized cells called microfold cells (
M cells) which sample antigen directly from the lumen and deliver it to
antigen-presenting cells (located in a unique pocket-like structure on their
basolateral side). Dendritic cells and macrophages can also directly sample the lumen by extending dendrites through transcellular M cell-specific pores. From this sampling, microbial components stimulate dendritic cells within Peyer’s patches. Once stimulated, the dendritic cells produce nitric oxide, retinoic acid, and IL-6, signals enhancing IgA isotype switching in B-cells, promoting IgA production within Peyer’s patches. At the same time the paracellular pathway of follicle-associated epithelium is closed tightly to prevent penetration of antigens and continuous contact with immune cells.
T cells,
B-cells and memory cells are stimulated upon encountering
antigen in Peyer's patches. These cells then pass to the
mesenteric lymph nodes where the immune response is amplified. Activated lymphocytes pass into the blood stream via the
thoracic duct and travel to the gut where they carry out their final effector functions. The maturation of B-lymphocytes takes place in the Peyer's patch. ==Clinical significance==