Curriculum · ENT, Ophthalmology, and Dermatology
Allergic contact dermatitis and the patch test
What it is
Contact dermatitis comes in two main types, and telling them apart is the point. Allergic contact dermatitis is a type IV, or delayed, hypersensitivity reaction, and it occurs 48-72 hours after exposure to the allergen. Irritant contact dermatitis is not allergic at all: it is direct tissue damage following a single exposure or multiple exposures to a known irritant, such as the weak acids or alkalis in detergents. Of the two, the irritant form is the common one and the allergic form is uncommon.
Causes and risk
Previous exposure is required for the allergic form and is not required for the irritant form, so an allergic reaction means the skin has met that allergen before. Cement is a frequent cause of contact dermatitis and can give either type: the alkaline nature of cement may cause an irritant contact dermatitis, while the dichromates in cement can cause an allergic contact dermatitis. This is why a painter who takes up new work may present with either one. Nickel is another well known allergen. The allergic form is often seen on the head following hair dyes, whereas the irritant form is often seen on the hands.