Contact allergy is an allergic skin reaction from contact with a substance that is usually harmless to others.
Figure 1. Allergic contact dermatitis to plaster.
Figure 2. Allergic reaction to flavine, spreading to other parts of the body.
The commonest manifestation is an itchy rash that develops over a few days, after skin contact with a substance. The affected area first becomes itchy, then red and swollen with vesicles (water bubbles).
Although usually confined to the area of contact, sometimes strikingly so (see Figure 1), the rash can spread to other parts of the body. (see Figure 2).
Occasionally the rash is more chronic, manifesting as an itchy patch that does not heal for weeks or even months. This usually occurs when there is frequent contact with the substance e.g. a watch strap . The rash resulting from contact allergy is called allergic contact dermatitis.
A substance that can cause contact allergy is called a contact allergen. Common contact allergens are:
Figure 3. Patch Test
A skin test called a patch test is used to confirm contact allergy. The upper back is used as a test site. A small amount of the suspected contact allergen (diluted to a non-irritant concentration) is applied onto the skin in an aluminium chamber and sealed with hypoallergenic tape. Usually, several suspected allergens are tested simultaneously. They are left on the skin for 48 hours and read at 48 and 96 hours. A positive reaction indicates that the patient is allergic to the test substance. This is a very safe procedure. (Figures 3 and 4).
Very often it is. An example of the need for a patch test is when a skin rash has persisted for a long time. Many skin care products and medication would have been tried. It is necessary to identify the cause of contact allergy. A patch test will help to ascertain the cause.
Another situation whereby a patch test is important is when a dermatitis is suspected to be due to work. There are many substances in the workplace that may cause contact allergy.
In many cases, removing the cause i.e. the allergen from skin contact will result in a cure.
Usually, once allergy to a substance has developed, it remains for life. That is to say, future contact with the substance will result in an allergic contact dermatitis again. This is exactly like a drug allergy. It is advisable for patients to carry a card to remind themselves and their doctors of their contact allergy.