When certain substances come in contact with your skin, they may cause a rash called contact dermatitis. Contact dermatitis can be divided into two types: allergic and irritant. Allergic contact dermatitis is caused by immune (allergy) cells in the skin reacting against the offending allergen. Allergic contact dermatitis is best known by the red, itchy and blistering rash seen after contact with poison ivy (also poison oak, sumac, mango) The reaction is caused by naturally occurring chemicals in the plants to which the majority of humans are allergic.
Typically the rash begins about 24 to 48 hours after contact, and will occur any place the plant, or anything the plant has touched, contacts the skin. Once a reaction starts, in can last anywhere from 7-28 days, even with treatment. With repeated exposure to the allergen, the time from contact to rash development shortens, and if there is frequent enough exposure, symptoms may begin a few hours after contact.
While reactions to plants are common, reaction to other allergens occur with less frequency, and occur more often in women than men. Chemicals that can cause allergic contact dermatitis include metals like nickel, which is found in most jewelry; also perfumes, dyes, rubber (latex) products and cosmetics. Some ingredients in topical medications also can lead to skin rashes. Neomycin, commonly found in topical antibiotic form, is an example, as is Benadryl cream, and even topical steroids, which are often used to treat rashes, may lead to allergic contact dermatitis. In fact, topical medicines in cream form are more likely to cause rashes, because the creams contain preservatives and other chemicals which can lead to allergy. Even products that have been used without problems for decades, can become allergens.
Treatment for allergic contact dermatitis depends to some extent on the severity of the reaction. In a severe case, oral steroids may be needed, but for less severe reactions, washing the site well to remove any of the offending agent; using cold compresses to reduce pain and blistering; oral antihistamines for control of itching; and short courses of topical steroids to reduce inflammation can all be useful treatments, especially for acute cases of allergic contact dermatitis.
Strict avoidance of the allergen, usually for life, is necessary, if the allergen has been identified, but in cases where the allergen is not known, use of Allergy Patch Testing can be critical.
The patch test involves applying various chemicals on the back for 48 hours. The patch is removed and interpreted after 48 hours and then read again at 72 to 96 hours. A positive test will show a small patch of redness, swelling or blistering at the site of a particular chemical. Unlike contact in nature, the reaction in patch testing is confined to a very small area, usually less than the size of a dime. Unfortunately, even with patch testing, the cause of some contact dermatitis remains unknown.
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