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The skin is the body's largest organ, and normally it helps keep "good stuff" like nutrients and water in, and keeps "bad stuff" like bacteria out. Sometimes, however, certain substances may cause an abnormal immune response in the skin, either through physical contact or as a result of ingesting something. This kind of reaction is known as skin allergies, which include any kind of reaction to something that results in a rash, hives, blisters, or other skin irritations. Skin allergies are affected by Langerhans' cells, which are special cells found in the epidermis (the outermost layer of skin) that are part of the skin's immune system. Normally, these cells help defend the body against infection, but sometimes they can be part of the development of skin allergies. The classification of "skin allergies" may include dermatitis (eczema), lichen planus, and various types of rashes or hives.