Haven't been on a lot, saw tag late.
Peyer's patches are normal to begin with, it's just a feature of the small intestine, it is the inflammation of them that is a sign of an issue.
Peyer's patches are tissue with immune cells. They're only found in the small intestine, you can see them with the naked eye through an endoscope, they look like little domes. The little dome bump is caused by the immune cells, the follicle. They're right on the surface of the intestinal wall, covered by M cells. I posted pictures of inflamed peyer's patches before. Peyer is from the person who discovered them, patches is because they're spread out like a patch. They're like mini lymph nodes in a way. Most people learn about lymph nodes long before they hear about peyer's patches so it's common to compare them to a lymph node.
Peyer's patches are like sensors, they're like the guards of the small intestine checking if everyone who is present in the small intestine is allowed to be there. If peyer's patches is the police station of the small intestine, then M cells are the gate that opens and closes. M cells are right on the surface, they actually aren't covered with mucus, and the guards at the M cells call in thousands of particles and antigens in to check for questioning in the peyer's patch.
The peyer's patch is filled with lymphocytes and dendritic cells. The immune system consists of the innate immune system and adaptive immune system. Dendritic cells awaken the adaptive immune cells if something has gone wrong (such as a salmonella infection), it's the alarm bell that will awaken those lymphocytes (B and T cells). They're often called antigen presenting cells, APC, since they present antigen to immune cells from the adaptive immune system as evidence that something is going horribly wrong in the body and they need new recruits to help, they're the alarm bell. Not only that, since those peyer's patches are conected to the lymphatic system, it will start recruiting more and more immune cells until the threat is over.
Anyway, why the peyer's patches are interesting in crohn's disease. Is because they are specific to the small intestine, if inflamed they are the first signs with an endoscope of crohn's disease, they are most active during teen years (crohn's is most diagnosed during those years) and we know AIEC invades peyer's patches through those M cells (AIEC is associated with crohn's disease).