Peyer’s patches are groupings of lymphoid follicles in the mucus membrane that lines your small intestine. Lymphoid follicles are small organs in your lymphatic system that are similar to lymph nodes.

Your lymphatic system is made up of tissues and organs containing white blood cells, which help your body fight infection. Your spleen, bone marrow, and lymph nodes are all part of your lymphatic system.

Peyer’s patches play an important role in immune surveillance of materials within your digestive system. Immune surveillance refers to the process by which your immune system recognizes and destroys potential pathogens.

Peyer’s patches are located in your small intestine, usually in the ileum area. The ileum is the last portion of your small intestine. In addition to further digesting the food you eat, the ileum also absorbs water and nutrients from food.

Most people have between 30 and 40 Peyer’s patches, and younger people tend to have more than older people. Researchers believe the number of Peyer’s patches in your ileum peaks in your 20s.

The size, shape, and overall distribution of Peyer’s patches varies from person to person.

Peyer’s patches have two important functions related to your immune system and how it responds to potential infections.

Response to infection

Peyer’s patches contain a variety of immune cells, including macrophages, dendritic cells, T cells, and B cells. There are also specialized cells, called M cells, next to your Peyer’s patches. These M cells feed antigens to the macrophages and dendritic cells of your Peyer’s patches. An antigen is a substance, such as a virus, that might produce a response from your immune system.

The macrophages and dendritic cells then show these antigens to your T cells and B cells, which determine whether or not the antigen requires an immune response. If they recognize the antigen as a harmful pathogen, the T cells and B cells in your Peyer’s patches signal your immune system to attack it.

Sometimes, bacteria and viruses can hack this mechanism and use it to enter the rest of your body through your small intestine.

Oral immune tolerance

Everything you eat eventually makes its way to your small intestine. Your body doesn’t recognize foods as foreign substances due to something called oral immune tolerance. This refers to the inhibition of immune responses to certain antigens. Your Peyer’s patches are frequently sampling material within your small intestine, so they likely play a role in determining which substances require an immune response.

No one’s sure about the exact role of Peyer’s patches in this process. A 2012 review noted a relevant study involving mice. Mice with reduced Peyer’s patch development had a harder time tolerating proteins as adults, but not other compounds. However, the same review also noted that other studies have concluded that not having Peyer’s patches doesn’t seem to affect oral immune tolerance.

Peyer’s patches likely play some kind of role in the development of oral immune tolerance, but researchers are still figuring out the details.

Bacterial infections

A variety of bacteria can invade your body by targeting M cells and Peyer’s patches. For example, a 2010 review noted that Listeria monocytogenes, which causes listeria, interacts with M cells and Peyer’s patches. The L. monocytogenes bacteria can:

  • migrate efficiently through M cells and rapidly move into the Peyer’s patches of mice
  • replicate within Peyer’s patches
  • move quickly from Peyer’s patches to other internal organs

Other types of bacteria known to do this include enterohemorrhagic Escherichia coli, which causes E. coli infections, and Salmonella typhimurium, which can cause food poisoning.

Viral infection

Viruses can also use M cells to enter your Peyer’s patches and start replicating. For example, researchers have observed that poliovirus, which causes polio, prefers to replicate within your small intestine.

Other viruses known to do this include HIV-1, which causes the most common type of HIV.

Crohn’s disease and ulcerative colitis

Crohn’s disease and ulcerative colitis are two types of inflammatory bowel disease. Crohn’s disease usually involves inflammation of your ileum, while ulcerative colitis typically involves your colon.

People with either Crohn’s disease and ulcerative colitis tend to have lesions on or around their Peyer’s patches, suggesting they likely play a role in the development of these conditions.

Prion diseases

Prions are pathogens that can change the shape or structure of proteins, especially those in the brain. Conditions involving prions are known as prion diseases. A common example is Creutzfeldt-Jakob disease, which is likely caused by the same prion responsible for mad cow disease in cows.

In many cases, prions are ingested with food, so they usually enter your small intestine before getting other parts of your body, such as your brain. Some studies have found large numbers of prions in the Peyer’s patches of several animal species. In addition, mice with fewer Peyer’s patches seem to be more resistant to prion diseases.

Peyer’s patches are small areas in your small intestine, especially the lower portion. Together with M cells, they play an important role in detecting pathogens in your digestive tract. However, Peyer’s patches might also play a role in the development of several conditions, including inflammatory bowel diseases, though this role isn’t well understood yet.