Peyer's patches are specialized lymphoid nodules found in the mucosa of the ileum, a part of the small intestine. They are a component of the immune system and play a crucial role in monitoring and defending against harmful pathogens that are ingested with food and drink. Peyer's patches contain large numbers of B-lymphocytes, T-lymphocytes, and macrophages, which work together to identify and eliminate potential threats. They also have a unique structure that allows them to sample and analyze the contents of the intestinal lumen, providing an early warning system for the immune system.

A patch test is a method used in clinical dermatology to identify whether a specific substance causes allergic inflammation of the skin (contact dermatitis). It involves applying small amounts of potential allergens to patches, which are then placed on the skin and left for a set period of time, usually 48 hours. The skin is then examined for signs of an allergic reaction such as redness, swelling or blistering. This helps in identifying the specific substances that an individual may be allergic to, enabling appropriate avoidance measures and treatment.

A transdermal patch is a medicated adhesive patch that is placed on the skin to deliver a specific dose of medication through the skin and into the bloodstream. It allows for a controlled release of medication over a certain period, typically lasting for 1-3 days. This method of administration can offer advantages such as avoiding gastrointestinal side effects, enabling self-administration, and providing consistent therapeutic drug levels. Common examples of transdermal patches include those used to deliver medications like nicotine, fentanyl, estradiol, and various pain-relieving agents.