Allergic Contact Dermatitis — Mechanism, Diagnosis, & Treatment

If you’ve ever been outdoors, chances are you or someone you know has encountered poison ivy. The rash from poison ivy is extremely itchy. Doctors can diagnose poison ivy from a mile away because of the characteristic symptoms and history. Poison ivy rash doesn’t develop right away. It builds up hours after the exposure and may not even develop on first contact. Dermatologists have a specific name for the rash caused by poison ivy — allergic contact dermatitis.


Allergic contact dermatitis is a special kind of allergic reaction in the skin. It occurs when the skin becomes sensitized to a foreign material, and the skin comes into contact with this material. This skin reaction will continue to flare as long as the skin is in contact with the material responsible for the reaction.

Mechanism behind Allergic Contact Dermatitis

Allergic contact dermatitis is a type 4 delayed hypersensitivity reaction. (1) Hypersensitivity reactions are when the immune system overreacts to an allergen. There are many types of hypersensitivity reactions.

Allergic contact dermatitis is classified as type 4 because it is mediated by the killer T cells of the immune system.

It is “delayed” because it takes time from the first exposure to the allergic material to the development of the reaction.

Symptoms of Allergic Contact Dermatitis

This rash is very itchy. The rash usually develops small fluid filled blisters that are characteristic of allergic contact dermatitis.

face paint skin allergies

The distribution of allergic contact dermatitis is unusual because it only develops in areas of the skin that come into contact with the allergy producing material. The rash will not be widespread. It will be circumscribed and sometimes follow precise patterns.

face paint skin allergies allergic contact dermatitis

Common Causes of Allergic Contact Dermatitis

Metals: Nickel is the metal that is most likely to cause allergic contact dermatitis.

Preservatives such as Methylisothiazolinone, Formaldehyde, Thiomersal, Iodine, and Parabens.

Fragrances such as Eugenol, Hydroxicitronellal, Cinnamic alcohol, and Cinnamic aldehyde.

Topical antibiotics such as neomycin. (2) This is why dermatologists often prefer Bacitracin alone for post-wound care instead of Neosporin.

How to Definitely Diagnose Allergic Contact Dermatitis

The only way to diagnosed allergic contact dermatitis with 100% diagnostic accuracy is to do patch testing. Patch testing is a painless procedure. A dermatologist will apply pre-made patches to the skin on the back.

These patches have different substances known to commonly cause allergies. The substances are numbered and spaced evenly apart. If the skin is allergic to a certain substance, the skin that is in contact with the chemical will react. The dermatologist can then pinpoint exactly what ingredient you may be allergic to.

Avoid Allergic Contact Dermatitis

It is impossible to predict what the skin will react to before it’s been exposed. But if you’re using a new hair dye, cosmetic product, sunscreen, or perfume, it may be wise to test the new substance on the inside of your wrist before using it on the rest of your body.

It’s better to be safe than sorry.

Treatment of Allergic Contact Dermatitis

This skin condition does not require treatment. Removing the exposure to the offending agent will resolve the rash on its own.


If the symptoms of itch are too bothersome, topical steroids and anti-histamines can help.

Have you ever suffered from allergic contact dermatitis due to poison ivy or another chemical? How did you treat it?


1.       DermNet New Zealand. Allergic Contact Dermatitis.

2.       Sheth VM, Weitzul S. Postoperative topical antimicrobial use. Dermatitis. 2008;19(4):181-9.

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