Peyer’s patches are clusters of lymphatic tissue in the small intestine and are an integral part of the gut-associated lymphoid tissue (GALT). As centers for immune surveillance, they monitor the intestinal environment to distinguish between harmful pathogens and harmless substances. This allows them to initiate appropriate immune responses to either attack invaders or tolerate innocuous materials.
Location and Structure of Peyer’s Patches
Peyer’s patches are primarily located in the ileum, the final section of the small intestine. Visibly, they appear as small, oval-shaped thickenings on the intestinal wall, often measuring a few centimeters in length. The number of these patches varies among individuals, typically ranging from 30 to 40, and their prevalence is highest in younger people. Microscopically, these structures are dense collections of lymphoid follicles, similar in composition to lymph nodes but lacking a surrounding connective tissue capsule.
These follicles are rich in immune cells, including B-lymphocytes situated in areas called germinal centers and T-lymphocytes in the surrounding parafollicular zones. A specialized layer of cells known as the follicle-associated epithelium (FAE) covers them. This epithelium contains distinctive cells called Microfold cells, or M cells. M cells are characterized by a folded surface structure adapted for capturing substances from the intestinal interior.
Immune Function in the Gut
The immune surveillance process begins with the M cells in the follicle-associated epithelium, which sample and transport antigens—small pieces of microbes like bacteria and viruses—from the intestinal lumen. These antigens are delivered directly to the underlying lymphoid tissue, which is populated with a high concentration of immune cells.
Once inside the patch, these antigens are presented to various immune cells, including T cells and B cells, initiating an immune response. Dendritic cells and macrophages process the antigens and present them to T cells, which then help to activate B cells. This activation stimulates the B cells to mature into plasma cells. These plasma cells produce a specific type of antibody called Secretory Immunoglobulin A (sIgA).
The sIgA is then released back into the intestinal lumen, where it functions as the first line of defense. This antibody can bind to pathogens and neutralize them before they have a chance to penetrate the intestinal wall and cause infection.
The Role in Oral Tolerance
Beyond defending against pathogens, Peyer’s patches perform the function of inducing oral tolerance. This is the process through which the immune system learns not to react to harmless substances, such as proteins from food and the vast populations of beneficial bacteria that reside in the gut. Without this mechanism, the body might launch an unnecessary and harmful immune attack against everyday dietary components or friendly microbes.
When antigens from food or commensal bacteria are taken up, the Peyer’s patch can initiate a pathway that leads to immune suppression rather than activation. This specialized response helps to prevent food allergies and inflammatory conditions that could arise from an overactive gut immune system.
This “educational” function is a balancing act. The immune cells within the patches must accurately differentiate between dangerous invaders and benign substances. This process of tolerance is fundamental to maintaining a peaceful coexistence with the microbes in the gut and preventing chronic inflammation.
Connection to Disease
The location and function of Peyer’s patches can be exploited by certain pathogens as a route of entry into the body. Microbes such as Salmonella, the poliovirus, and infectious proteins known as prions have evolved to specifically target the M cells. Using these cells as a gateway, pathogens can bypass the initial gut defenses and invade the underlying tissues, leading to systemic infection. Prions, which are responsible for conditions like Creutzfeldt-Jakob disease, can replicate within Peyer’s patches after being ingested.
Dysfunction within Peyer’s patches is also implicated in chronic inflammatory conditions, most notably inflammatory bowel disease (IBD), which includes Crohn’s disease. In individuals with these conditions, the immune-regulating balance within the patches can be disrupted. This can lead to an overactive and persistent immune response directed against harmless gut contents, such as food particles or beneficial bacteria. The Peyer’s patches, instead of maintaining immune balance, become sites that perpetuate the inflammatory cycle. This sustained inflammation can cause significant damage to the lining of the intestine over time.