In most instances, people with allergies have an immediate reaction when their body comes in contact with or ingests certain allergens. Someone that is peanut allergic will have an immediate reaction if they mistakenly ingest peanuts. Someone that is allergic to cats may immediately start sneezing when they come in contact with cats.
There are other allergens that can produce a delayed response in people that are allergic. This reaction may occur within two to four days after the initial contact. This type of allergic reaction typically occurs when the person's skin comes in contact with a particular allergen. Skin irritation may develop in the area of the initial contact then spread to other parts of the body. This rash is referred to as contact dermatitis.
Common allergens that produce a delayed hypersensitivity include:
Testing for delayed hypersensitivity is performed using a "patch test." A panel of 65 different allergens is placed on the patient's back and secured with tape. The patient returns in 48 hours when the panel is removed and any skin reactions are noted. The patient returns again in another 48 hours when skin reactions are documented again. Results are reviewed with the patient and avoidance measures discussed.