Definition/Introduction

The human immune system is a complicated marvel that has evolved to demonstrate wide distribution through all the systems of the body. For example, the skin has the Langerhans cells; the liver has the Kupffer cells, the adenoids of the nasopharynx, etc. One such improvisation of the immune system is the MALT- the mucosa-associated lymphoid tissue. The MALT of the gut is the gut-associated lymphoid tissue(GALT). This article is about a part of the GALT- the Peyer's patches.

Peyer's patches are a group of well-organized lymphoid follicles located in the lamina propria and submucosa of the distal portion of the small intestine - the ileum and jejunum and sometimes in the duodenum. Almost 50% of these patches are in the distal ileum. These patches are the private immune system of the gut that helps in identifying the antigens and in producing antibodies. They grow and increase in number to a maximum at around 15 to 25 years of age, after which they decrease in number as age increases.[1]

The Peyer's patches organize in the form of follicles, which mainly consist of B cells. However, the other immune cells therein are T cells, plasma cells, mast cells, eosinophils, macrophages, and basophils. In the Peyer's patches, the B lymphocytes reside in the germinal centers of these follicles where their maturation takes place, and the T lymphocytes are in the parafollicular zone. The Peyer's patches at their apices contain M (microfold) cells, enterocytes expressing MHC class II. They endocytose the particulate form of antigens and take them to the antigen-presenting cells (APCs). One of the four main functions of APCs is to shuttle the antigens from tissues to sites of lymphocyte priming, one of which is the Peyer's patches. 

Issues of Concern

The special feature of Peyer's patches is that these lymphoid follicles are surrounded by an epithelium known as the follicle-associated epithelium(FAE).[2] This epithelium contains a decreased number of intestinal villi, a thin mucosa layer, and characterized by the presence of epithelial M cells called the microfold cells.[3] These M cells are the antigen-presenting cells of the Peyer's patches and thus help in their uptake and transport across the intestinal lumen into the bloodstream. The porous nature of the basal lamina of the FAE helps in this antigen uptaking process.[4]

The Peyer's patches contain a significant number of dendritic cells, macrophages, and lymphocytes. Since the gastrointestinal system has exposure to a significant number of pathogens, it is vital for immune surveillance. Once a pathogen is encountered by the M cells in the epithelium, they are presented to the dendritic cells and the macrophages across the membrane via the porous basal lamina, primarily via the transcellular M cell-specific pores.[5] These cells, in turn, activate the lymphocytes, which then travel to the mesenteric lymph nodes, and the amplification of this immune process takes place. The B lymphocytes in the germinal centers are differentiated into IgA secreting plasma cells. The produced IgA, along with a protective secretory layer, is then secreted into the lumen to deal with the intraluminal pathogen.

T cells n Peyer's patches function similarly to LPLs (lipoprotein lipase) and other circulating T cells. However, the distinct feature of these T cells is to secrete tolerogenic cytokines like TGF-beta and  IL-10 on exposure to common food allergens, as seen in some animal experiments.

Clinical Significance

In Salmonella typhi infection, intestinal necrosis occurs at the level of the Peyer's patches, thereby causing perforation of the distal ileum (in the antimesenteric bowel wall)most commonly, that occurs during the third week of febrile illness.[6] Peyer's patches are also culprits in the development of idiopathic intussusception.

Intussusception most commonly occurs in infants, especially during the weaning period & also due to viral infection like adenovirus in early childhood. This presentation suggests that the exposure of the infant's gut to new pathogens causes the hypertrophy of the Peyer's patches at the lymphoid-rich terminal ileum, resulting in recurrent intussusception or intestinal obstruction.[7][8]

Increased numbers of the Peyer's patches also correlate with the development of prions disease, with the M cells uptaking the orally invaded prions and the Peyer's patches acting as replicating centers of the prions.[9]

However, Peyer's patches keep the intestinal flora at appropriate levels and the pathogens at bay, thereby preventing a large number of infections.

As a part of the pathogenesis of food allergy, the Peyer's patches and vasculature of lamina propria allow the circulation of inflammatory cells to the immune structures.

Nursing, Allied Health, and Interprofessional Team Interventions

The pathologies associated with the functioning and deranged morphology of Peyer's patches are important in many gastrointestinal diseases, as seen in the clinical significance. Hence, the appropriate diagnosis and management of these pathologies deal with their timely diagnosis and management. It involves the combined work of the interprofessional team, which includes primary health care clinicians, pathologists/histopathologists, infectious disease specialists, gastroenterologists, and immunologists.