Allergic contact dermatitis , also called contact allergy, is a type of contact dermatitis that causes itchy and red skin rash when the skin comes in contact with a substance that triggers an immune response.

Many different substances can cause allergic contact dermatitis. Usually, these substances cause no trouble for most people, and may not even be noticed the first time the person is exposed. However, once the skin becomes sensitive or allergic to the substance, any exposure will produce a rash.

When a foreign substance comes in contact with the skin, it is linked to skin proteins, forming an allergen-protein complex that leads to sensitization. Upon re-exposure of the epidermis to the allergen, the sensitized T cells initiate an inflammatory cascade, causing the skin changes associated with allergic contact dermatitis.

Symptoms of allergic contact dermatitis vary depending on the cause, and the same person may have different symptoms over time. The symptoms are very similar to the ones caused by irritant contact dermatitis, which makes the first even harder to diagnose. The first sign of allergic contact dermatitis is the presence of a rash or skin lesion at the site of exposure.

Sometimes, it is easy to recognize allergic contact dermatitis and no specific tests are necessary. Taking a very good history including information on work environment, hobbies, products in use at home and work, and sun exposure will enhance the chances of finding a diagnosis. For more difficult cases, dermatologists will perform patch tests in patients with suspected contact allergy, particularly if the reaction is severe, recurrent or chronic. The tests can identify the specific allergen causing the rash.

The type of treatment depends on the cause of contact dermatitis. Common treatments include oral and\or topical corticosteroids, antihistamines, lotions and creams.

According to the US National Eczema Association, allergic contact dermatitis is the second most common type of contact dermatitis. The first being irritant contact dermatitis.

Allergic contact dermatitis is an allergic reaction to a material called an allergen that comes in contact with the skin. Allergic contact dermatitis causes an itchy and weeping rash localized to the area in contact with the allergen. It is a delayed hypersensitivity reaction that usually develops two or more days after contact with the allergen, and lasts as long as contact continues and for a short time afterwards.  The allergen is harmless to people that are not allergic to it.

Allergic contact dermatitis is not classified into subtypes, but is of itself one of two subtypes of contact dermatitis, the other being irritant contact dermatitis.

Allergic contact dermatitis occurs when an allergen triggers an immune reaction in the skin. It usually affects only the area that were exposed to the allergen. However, it may be triggered by something that enters your body through foods, flavorings, medicine, or medical or dental procedures.

According to the American Academy of Dermatology, there are more than 3,600 substances that can cause allergic contact dermatitis. Common allergens include:

• Nickel: Used in jewelry, buckles and many other items.
• Balsam of Peru: Used in many products, such as perfumes, cosmetics, mouth rinses and flavorings.
• Formaldehyde: Commonly used in adhesives, solvents and other products.
• Urushiol: Found in some plants such as poison ivy and mango.

Common products that may contain allergens include:

• Personal care products: Such as deodorants, body washes, hair dyes, cosmetics and nail polish.
• Airborne substances: Such as those used in aromatherapy and spray insecticides.
• Topical medicines: Such topical corticosteroids in rare cases.

Children develop the condition from the usual offenders and also from exposure to car seats and the plastic in toilet seats.

Having a history of any allergy increases the risk of developing allergic contact dermatitis. People with existing skin disorders like eczema, psoriasis and acne are also more likely to get allergic contact dermatitis. Allergic contact dermatitis is caused by the direct application of the inciting substance to unprotected skin; therefore, the key risk factor is exposure to that substance.

Allergic contact dermatitis is an immunological reaction that occurs in people who have been previously exposed to an allergen. When a person's skin encounters an allergen for the first time, the allergen enters the outermost layer of skin known as stratum corneum and binds to carrier proteins. The allergen-protein complex is engulfed by Langerhans cells in the epidermis and subsequently processed. Then, the Langerhans cells travel to nearby lymph nodes and present the processed allergen-protein complex to naive Th1 cells. This leads to the release of interleukin-1 and 2, which initiate clonal proliferation of newly sensitized Th1 cells as well as the release of memory Th1 cells into circulation.

Upon re-exposure to the allergen, the circulating memory Th1 cells use their skin-specific homing receptors to enter the skin at the site of allergen exposure and release inflammatory cytokines that lead to the response typically seen in allergic contact dermatitis. The reaction most often occurs 24 to 48 hours after the exposure.

In rare cases, the person may become sensitized to a strong allergen such as poison ivy after a single exposure. Nevertheless, weaker allergens may require multiple exposures over several years to trigger an allergy.

Signs and symptoms of allergic contact dermatitis vary depending on the cause, and the same person may have different symptoms over time. These may include:

• Itching that may be severe
• Rash, which may not appear until 24 to 48 hours after exposure.
• Excessively dry skin
• Burning and stinging
• Hives (round welts on the skin that itch intensely)
• Fluid-filled blisters
• Oozing blisters that leave crusts and scales

The key to successful treatment of contact dermatitis is identifying what is causing the symptoms. Doctors rely on these main steps to determine the cause:

Medical history and physical exam

Taking a thorough medical history, including information on work environment, hobbies, products in use at home and work and sun exposure will enhance the chances of reaching the correct diagnosis.

Patch test

Dermatologists will perform patch tests in patients with suspected contact allergy, particularly if the reaction is severe, recurrent or chronic. Patch testing is an important tool to diagnose allergic contact dermatitis.

In this test, selected chemicals are put on an adhesive strip or patches and applied to the back of the patient, where there are no symptoms currently present. The patches are left on for 48 hours. After 48 hours, the doctor removes the patches and looks at the skin for any signs of reaction. After two more days, the doctor looks at the patch sites for signs of inflammation. If there is inflammation, the allergy to that particular chemical is confirmed.

Other tests

The doctor perform tests like skin lesion biopsy or skin lesion culture to rule out other possible causes for the symptoms, such as psoriasis or infections.

The doctor will recommend treatment based on what is causing the problem. Medications should not be used in amounts more than prescribed or used more often than advised. The key treatments for contact dermatitis include:

Allergen avoidance

The key to this after identifying what is causing the rash is staying away from it as much as possible. The healthcare provider will provide the patient with a list of products that contain the substance that causes the effects.

Steroid creams

Steroid creams, such as topical corticosteroids, may be prescribed to treat allergic contact dermatitis.

Immune modulating drugs

Tacrolimus ointment and pimecrolimus cream may be used to reduce inflammatory skin reactions in the treatment of allergic contact dermatitis. However, the Food and Drug Administration has warned about a possible link between these drugs and lymphoma and skin cancer.

Oral medications

In severe cases, the doctor may prescribe oral corticosteroids to reduce inflammation, antihistamines to relieve itching or antibiotics to treat a bacterial infection of complicated cases.

Possible complications of allergic contact dermatitis include the following:

Autoeczematization: Severe reactions may generalize to become widespread or affect other parts of the body away from the primary spot of inflammation, a condition known as autoeczematization or “id” reaction.

Erythroderma: Allergic contact dermatitis can also lead to erythroderma, which widespread reddening of the skin. If this condition is accompanied with skin peeling, it is then called exfoliative dermatitis.

Neurodermatitis: Neurodermatitis is a skin condition that leads to chronic itchy and leathery skin. It starts with an of itchy patch of skin that, when scratched, becomes itchier. Eventually, the patient may scratch out of habit causing the skin to become discolored and thick.

Infections: Scratching rashes resulting from allergic contact dermatitis may cause the skin to ooze and become wet, creating a suitable place for bacteria or fungi to grow and cause an infection.

There is no known way that can prevent someone from developing an allergy, but there are several methods that can help avoid the allergen to prevent an allergic reaction:

Wear protective clothing: Wearing protective clothing, such as facemasks, goggles and gloves help reduce the chance of an allergen coming in contact with the skin.

Find alternatives: Check the ingredients on commonly used products, such as make-up and soap to make sure they do not contain any irritants or allergens, and find alternatives that contain none of the substances that irritate your skin.

Apply a barrier cream or gel: Barrier creams or gels are products that provide a protective layer over the skin, which prevents allergens from touching it.

Take care around pets: Plant allergens, such as poison ivy, can be easily spread to people through pets.

Use moisturizer: This keeps the skin from drying and helps protect it from allergens.

Wash the skin: If an allergen comes in contact with the skin, washing it with water removes most of the rash-causing substance.

Allergic contact dermatitis clears up without complications in 2 or 3 weeks in most cases; however, it may return if the allergen causing the reaction is never identified or cannot be avoided.

Sometimes, people need to change jobs or job habits if the disorder is caused by exposure at work that cannot be avoided.

According to the US National Eczema Association, allergic contact dermatitis is the second most common type of contact dermatitis.

A review conducted by the Gentofte University Hospital in Denmark showed that one-fifth of the population in Europe suffers from contact allergy to at least one contact allergen. Most common are allergies to nickel, fragrances and preservatives. In addition, contact dermatitis occurs twice as frequently in women, and often starts at a young age, with a prevalence of 15% in 12–16 year olds.