The cause is a disorder of the immune system called Delayed Hypersensitivity. The person need to have an initial exposure to the invading substance (allergen), then the immune system will build up reaction against the allergen from a small number of ‘memory’ cells that can recognise the allergen. This “sensitisation” process takes about 7 to 10 days. Then when a repeated exposure is met, over 2 to 3 days these memory cells give rise to a whole cascade of responses that result in dermatitis.

Once this sensitisation is stimulated, the delayed hypersensitivity will forever exist, and after every episode of repeated exposure to the same substance, contact allergic dermatitis will happen again.

Unlike irritant contact dermatitis, allergic contact dermatitis does not require a concentration above a certain level. Only a minute quantity is sufficient to mount the allergic responses.

Contrary to common belief, people with atopy, such as atopic eczema, actually are less easy to develop allergic contact dermatitis. It is probably because they need to apply many different topical creams, that they have higher incidences of this.

Many commonly seen allergic contact dermatitis are due to occupational exposure to industrial substances. The patient usually notices improvement when he or she is away from work. This diagnosis has implications in litigation and compensation, and requires the opinions of occupational physicians and dermatologists.

Common Industrial Allergens

1. Cement , leather, matches : contains potassium dichromates
2. Rubber, colophony, adhesives, dyes : contain epoxy resins

Common Non-industrial Allergens

1. Skin products : perfumes, nail lacquer, cosmetics, creams. Contain allergens such as preservatives, fragrances (e.g. Balsam of Peru), lanolin, cetearyl alcohol, neomycin, sunscreen ingredients.
2. Hair products : contain paraphenylenediamine
3. Rubber：gloves, shoes, rubber band inside trousers, rubber support of glasses. Contain epoxy resins.
4. Metals : watches, ear rings, necklaces, metal glasses frames, belt buckles, metal stubs in trousers. Usually due to nickel or cobalt.
5. Dyes : clothings, hair dyes. Contain also epoxy resins.
6. Plants :
   - Primula : flowers and leaves contain quinone, can cause very acute dermatitis with blisters
   - Rhus such as poison oak and poison ivy : also cause an acute dermatitis
   - Chrysanthemum : the leaves contain lactone, cause a chronic dermatitis with dry hard skin.
   - Bulb such as onion and garlic : contain a natural anti-fungal element, usually cause chronic dermatitis of the finger tips.
7. Others：matches. Contain potassium dichromate

This is by no means a comprehensive list of all possible allergens that can cause contact eczema. Some of the materials known to potentially cause skin reactions are themselves components of common treatments for eczema, for example lanolin in moisturisers and hydrocortisone in steroid cream. Therefore if the condition is worsening or proving resistant to treatment, the possibility that the treatment itself is contributing to the skin problem needs to be considered.

Allergic Reaction to Shoe Leather

Allergic Reaction to Skin Cream

Signs

The rash first appear at the site of contact, but later often spreads out to a wider area. Sometimes the cause and effect relationship is obvious, as in the case of belt buckle and dermatitis around the umbilicus. At other times, especially if a long time has passed since the initial exposure, it may be impossible to identify the cause.

Mild cases only show little redness and scaling. More severe acute cases will have swelling, weeping, blisters, crusting, or even pus formation. Chronic cases will become dry, hard, thickened, and scaling.

Management & Treatments

The first thing to do is to avoid the allergen. If this is possible, then the problem is quite likely to resolve. Usually moisturisers and steroid cream is required. Strong preparations may be needed if the condition is more severe and medical attention is advised. Occasionally oral steroid is needed for acute severe dermatitis.

The extent of improvement depends on the degree of dermatitis and the frequency of exposure to the allergen. If the offending substance is repeatedly contacted, then even a lot of treatments will end up with little effect.

It may be difficult to distinguish allergic (which is an immune response) from irritant (which is a direct skin irritation) contact dermatitis. In practice, this difference is not so important because the same lines of treatment are required for both. If a search is needed to identify a possible allergen, then a patch test can be done.

The new cytokine inhibitors creams, such as pimecrolimus and tacrolimus, may be as effective as steroid creams, although their usage in this area has been short and they are still not the first line treatments yet.