What Is an Id Reaction?

Skin Reaction Can Reveal Much About Your Health

Contact Dermatitis. Credit: BSIP / Contributor / Getty Images

An interface dermatitis (id) reaction, also known as autoeczematization, is an itchy rash with small, water-filled blisters that is caused in response to a primary infection, usually fungal.  It most often occurs on the sides of the fingers but can also be found on the chest or arms. 

What Does an Id Reaction Look Like?

An id reaction can present itself in a number of ways. Examples include:

  • localized or widespread vesicular lesions (small, fluid-filled spots)
  • red patches with raised red bumps (maculopapular or scarlatiniform eruptions)
  • deep, raised, bruise-like areas on the shins (erythema nodosum)
  • pinkish red spots that resemble targets (erythema multiforme)
  • light pink teardrop-shaped bumps (guttate psoriasis)

What Causes an Id Reaction?

The most common cause of id reactions is a fungal infection somewhere else on the body, especially athlete's foot. It can also occur as a result of a bacterial, viral, or parasitic infection.

Id is also typically seen in autoimmune skin disorders such as lichen planus or cutaneous lupus erythematosus but can also occur as result of an immune response to viruses, tumors, or even drugs. One study, in fact, found that id reactions were common in bladder cancer patients undergoing BCG therapy.

How Is an Id Reaction Diagnosed and Treated?

Treating an id reaction depends largely on what caused it in the first place. Your dermatologist will want to identify the underlying infection and/or conditions that spurred the response.

This is usually done by taking scrapings from the affected area.

Other tests can include:

  • skin culture for bacteria, fungus, or virus
  • allergy skin testing to rule out contact dermatitis
  • Skin biopsy, usually performed to confirm a diagnosis

As id reactions as thought to be an allergic response to fungi or other pathogens, treatment of the underlying infection will typically resolve the rash.

In some cases, an id reaction may the only way a person knows that he or she has an infection which need to be treated. To relieve symptoms, your doctor may prescribe a corticosteroid cream or oral anti-itch medication. Antihistamines may be prescribe in cases of an allergic response.

Common Infections That Cause Interface Dermatitis

While an id reaction may occur as a result of any number of reasons, the more common causes include:

While an id reaction rash cannot be passed from one person to the next, the primary condition that gave rise to the reaction may be contagious.


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Lowther C.; Miedler, J.; Cockerell, C.; et al. “Id-like reaction to BCG therapy for bladder cancer.” Cutis. March 2013; 91(3):145-151.

Wenzel, J., and Tüting, T. “An IFN-associated cytotoxic cellular immune response against viral, self-, or tumor antigens is a common pathogenetic feature in "interface dermatitis." J Invest Dermatol. Oct 2008; 128(10):2392-2402. 

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