There are 2 types of allergy skin testing. The first is called the skin prick test, usually performed on the arms, which is for type 1 allergies. These reactions occur within seconds to minutes after exposure to allergens and include sensitivities to airborne particles like pollen, dust mites and animal dander, and foods like peanuts, milk, eggs, wheat and soy. This category of allergies causes hives and eczema, also called atopic dermatitis. Type 1 skin reactions begin inside the body and may affect the skin all over the body, even where the substance was not in contact.
The second type of allergy skin testing is patch testing, usually performed on the upper back. Patch testing is for type 4 allergies, also called delayed hypersensitivity or allergic contact dermatitis, which occurs hours to days after exposure. An example of a type 4 allergy is poison ivy. In cases of allergic contact dermatitis, the rash is located where the offending substance came in contact with the skin, and not elsewhere. There are several dozen substances responsible for allergic contact dermatitis that are commonly found in our environment, including nickel, fragrance ingredients, neomycin, latex and formaldehyde. Patch testing will help identify the substances that cause this type of dermatitis in a particular person.
