Contact allergens are substances that can cause a bad reaction by involving the immune system after penetrating the skin or mucosa. Most contact allergens that cause dermatitis are chemicals rather than natural proteins whereas most allergens that cause contact urticaria are proteins. Allergic contact dermatitis is investigated by patch testing whereas contact urticaria is investigated by skin prick testing. Patch testing involves applying patches containing different chemicals to the back for 48 hours and then looking for eczematous reactions beneath them. The patches usually consist of a panel of allergens that are standardized internationally but it may be possible to test commercial products that are thought to have caused allergic contact dermatitis in the same way. Skin prick testing involves pricking a solution of the suspected allergen into the skin and then looking for an immediate itchy, red weal reaction.
The commonest allergen that causes contact dermatitis is nickel. Up to 30% of women may be allergic to it and the number of men who react is increasing as fashions change. Cheap jewellery containing nickel is a common reason for sensitization but a number of occupations, including hairdressing and machining, put people at higher risk. Nickel has to leach out of the metal and penetrate the skin before it can cause an allergic reaction so this usually happens where there is close and prolonged skin contact. Cobalt and potassium dichromate may also be present in cheap metal so it is common to find that these cause positive patch test reactions as well. Fortunately pure gold, platinum and silver are very rare allergens so more expensive jewellery can usually be worn safely. Other common contact allergens include chemicals that are used during the manufacture of rubber from natural latex, permanent hair dyes, biocides that are used in cosmetics, toiletries and some dermatological creams. Sometimes antibiotics can become contact allergens, especially e.g. aminoglycosides that are still used in nose, ear and eye drops. Fragrances are quite common sensitizers but, because there are many different fragrances that can be combined to make a particular perfume, it is often possible to find products that do not cause a reaction. Less common contact allergens include certain plants, glues and even steroid molecules that can be applied to the skin to treat contact dermatitis.
Allergens that can cause contact urticaria include latex, animal saliva, caterpillars, meat, eggs, plants and even human excretions, including seminal fluid.