Rachel L. Schreiber, MD, an allergist/immunologist at Schreiber Allergy in Rockville, MD, discusses contact dermatitis and patch testing. Contact dermatitis is a type 4 hypersensitivity reaction, which is characterized by a delayed reaction. Patch testing is used to figure out if you’re reacting to certain substances that come in contact with your skin. Results from the patch test provide information about what products should be avoided.