Peyer's patches are areas of specialized tissue in the lower area of the small intestine that work to distinguish friend from foe as food passes through the gastrointestinal tract. Sensitized cells inside these areas identify antigens and decide whether they are harmless, associated with foods that the person is consuming for nutrition, or harmful, and linked with organisms like bacteria that could try to colonize the body. The Peyer's patches pass the word to the rest of the intestinal tract, facilitating either digestion and uptake of nutrients or an immune system attack on an invader.
There are 30 to 40 Peyer's patches, all containing lymphoid tissue that is high in white blood cells. Other lymphoid tissue, known as gut-associated lymphoid tissue (GALT), can be found in other areas of the intestines. When people are young, these areas are highly visible and easy to distinguish. With aging, they tend to fade and become less prominent.
The Peyer's patches take the form of small ovoid nodules that line the gut. As food passes by, the antigens in the food are presented to the tissue and it determines whether it recognizes them and how they are classified. In addition to recognizing antigens and triggering the appropriate response, the Peyer's patches can also learn to identify new antigens, storing this information for future reference to make the immune system more effective.
Sometimes the immune system becomes confused about the identity of antigens it interacts with. It may mistakenly think that an antigen like a protein found in peanuts is actually harmful, and trigger a response when this antigen is identified. The response leads to inflammation as the confused immune system attempts to neutralize what it thinks is a harmful invader. Food allergies can result in reactions varying from mild discomfort in the gut to life-threatening shock caused by an over-eager immune system.
The immune system treats the gut as outside the body, despite the fact that it is firmly and clearly positioned in the abdomen. It is considered a point of contact with the outside world because everything people eat and drink will make its way into the gut. This creates risks, as people may consume bacteria, viruses, fungi, and other harmful organisms along with the things they mean to be eating. The plethora of GALT in the gut is designed to provide a first line of immune defense to prevent harmful organisms from entering the body.