Allergic contact dermatitis (ACD) is a form of contact dermatitis that is the manifestation of an allergic response caused by contact with a substance; the other type being irritant contact dermatitis (ICD).
Although less common than ICD, ACD is accepted to be the most prevalent form of immunotoxicity found in humans. By its allergic nature, this form of contact dermatitis is a hypersensitive reaction that is atypical within the population. The mechanisms by which these reactions occur are complex, with many levels of fine control. Their immunology centres on the interaction of immunoregulatory cytokines and discrete subpopulations of T lymphocytes.
Allergic contact dermatitis is a type 4 or delayed hypersensitivity reaction and occurs 48–72 hours after exposure to the allergen. The mechanism involves CD4+ T-lymphocytes, which recognise an antigen on the skin surface, releasing cytokines that activate the immune system and cause the dermatitis. Note:
- Contact allergy occurs predominantly from an allergen on the skin rather than from internal sources or food.
- Only a small number of people react to the specific allergen, which is harmless to those who are not allergic to it.
- They may have been in contact with the allergen for years without it causing dermatitis.
- Tiny quantities of an allergen can induce dermatitis.
- Patients with impaired barrier function of the skin are more prone to allergic contact dermatitis, eg patients with leg ulcers, perianal dermatitis, or chronic irritant contact dermatitis.
- Patients with atopic dermatitis associated with defective filaggrin (a structural protein in the stratum corneum) have a high risk of also developing allergic contact dermatitis.
Individuals in the following profession are at an increased risk of Allergic Contact Dermatitis:
- Cooks and chefs
- Glass blowers
- Factory workers
- Hair stylist
- Constructions workers
- Healthcare workers
It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases ones chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.
Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.
Contact dermatitis usually occurs on areas of your body that have been directly exposed to the substance — for example, along a calf that brushed against poison ivy or under a watchband that triggers an allergy. The reaction usually develops within minutes to hours of exposure to an irritating substance or allergen. The rash can last two to four weeks.
Signs and symptoms of contact dermatitis include:
- Red rash or bumps
- Itching, which may be severe
- Dry, cracked, scaly skin, if your condition is chronic
- Blisters, draining fluid and crusting, if your reaction is severe
- Swelling, burning or tenderness
The severity of the rash depends on:
- How long you’re exposed
- The strength of the substance that caused the rash
- Environmental factors, such as temperature, airflow and sweating from wearing gloves
- Your genetic makeup, which can affect how you respond to certain substances
The key to successful treatment of contact dermatitis is identifying what’s causing your symptoms and figuring out whether you have the irritant or allergic type. Doctors rely on these main steps to determine the cause:
- A thorough medical history and physical exam. Your doctor may be able to diagnose contact dermatitis and identify its cause by talking to you about your signs and symptoms, questioning you to uncover clues about the culprit, and examining your skin to note the pattern and intensity of your reaction.
- A patch test. Your doctor may recommend a patch test (contact delayed hypersensitivity allergy test) to see if you’re allergic to something. This test can be useful if the cause of your rash isn’t apparent or if your rash recurs often.
You may be asked to avoid certain medications and sun tanning your back for a week or two before the test.
During a patch test, small amounts of potential allergens are applied to adhesive patches, which are then placed on your skin. The patches remain on your skin for two days, during which time you’ll need to keep your back dry.
Your doctor then checks for a skin reaction under the patches and determines whether further testing is needed. Often, people react to more than one substance.
Key treatments for contact dermatitis include:
- Avoiding the irritant or allergen. The key to this is identifying what’s causing your rash and then staying away from it. Your doctor may give you a list of products that typically contain the substance that affects you.
- Applying prescription steroid creams. If self-care measures haven’t worked, your doctor may prescribe a steroid cream.
- Applying medications to repair the skin. You can help repair the skin and prevent relapse with creams and ointments containing drugs that affect the immune system, such as calcineurin inhibitors tacrolimus (Protopic) or pimecrolimus (Elidel). This solution is recommended for long-term treatment of contact dermatitis. But the Food and Drug Administration has warned about a possible link between these drugs and lymphoma and skin cancer.
- Using oral medications. In severe cases, your doctor may prescribe oral corticosteroids to reduce inflammation, antihistamines to relieve itching or antibiotics to fight a bacterial infection.
General prevention steps include the following:
- Avoid irritants and allergens. Try to identify and avoid substances that irritate your skin or cause an allergic reaction.
- Wash your skin. You might be able to remove most of the rash-causing substance if you wash your skin right away after contacting it. Use a mild, fragrance-free soap and rinse completely. Also wash any clothing or other items that may have come into contact with a plant allergen such as poison ivy.
- Wear protective clothing or gloves. Face masks, goggles, gloves and other protective items can shield you from irritating substances, including household cleansers.
- Apply an iron-on patch to cover metal fasteners next to your skin. This can help you avoid a reaction to jean snaps, for example.
- Apply a barrier cream or gel. These products can provide a protective layer for your skin. For example, an over-the-counter skin cream containing bentoquatam (IvyBlock) may prevent or lessen your skin’s reaction to poison ivy.
- Use moisturizer. This can help restore your skin’s outermost layer and keep your skin supple.
- Take care around pets. Pets can easily spread to people allergens from plants such as poison ivy.
To help reduce itching and soothe inflamed skin, try these self-care approaches:
- Avoid allowing the reaction-causing substance to touch your skin. If it’s a piece of jewelry, you may be able to wear it by putting a barrier between you and the metal. For example, line the inside of a bracelet with a piece of clear tape or paint it with clear nail polish.
- Apply an anti-itch cream or calamine lotion to the affected area. A nonprescription cream containing at least 1 percent hydrocortisone can temporarily relieve your itch.
- Take an over-the-counter anti-itch drug. A nonprescription oral antihistamine, such as diphenhydramine (Benadryl, others), may be helpful if your itching is severe.
- Apply cool, wet compresses. Moisten soft washcloths and hold them against the rash to soothe your skin for 15 to 30 minutes. Repeat several times a day.
- Avoid scratching. Trim your nails. If you can’t keep from scratching an itchy area, cover it with a dressing and bandage.
- Soak in a comfortably cool bath. Sprinkle the water with baking soda or an oatmeal-based bath product (Aveeno, others).
- Wear smooth-textured cotton clothing. This helps avoid irritation.
- Choose mild soaps without dyes or perfumes. Rinse completely, pat your skin dry and apply moisturizer.
- Protect your hands with moisturizers and gloves. Reapply moisturizers throughout the day. And choose gloves based on what you’re protecting your hands from. For example, plastic gloves lined with cotton are good if your hands are often wet.
The prognosis depends on how well the affected individual can avoid the offending allergen. 
Individuals with allergic contact dermatitis may have persistent or relapsing dermatitis, particularly if the material(s) to which they are allergic is not identified or if they continue to practice skin care that is no longer appropriate (ie, they continue to use harsh chemicals to wash their skin, they do not apply creams with ceramides or bland emollients to protect their skin).
The longer an individual has severe dermatitis, the longer it is believed it will take the dermatitis to resolve once the cause is identified.
Some individuals have persistent dermatitis following allergic contact dermatitis, which appears to be true especially in individuals allergic to chromates.
A particular problem is neurodermatitis (lichen simplex chronicus), in which individuals repeatedly rub or scratch an area initially affected by allergic contact dermatitis.
Death from allergic contact dermatitis is rare in the United States. Allergic contact dermatitis to the weed wild feverfew caused deaths in India when the seeds contaminated wheat shipments to India. This plant then became widespread and a primary cause of severe airborne allergic contact dermatitis.
Allergic contact dermatitis starts as a localised reaction to an allergen in contact with the skin, but severe reactions may generalise due to autoeczematisation, and can lead to erythroderma.
Ingestion of a contact allergen may rarely may lead to baboon syndrome or generalised systemic contact dermatitis.