Allergic contact dermatitis
The mechanism is that of delayed (type IV) hyper-sensitivity. It has the following features.
• Previous contact is needed to induce allergy.
• It is specific to one chemical and its close relatives.
• After allergy has been established, all areas of skin will react to the allergen.
• Sensitization persists indefinitely.
• Desensitization is seldom possible.
In an ideal world, allergens would be replaced by less harmful substances, and some attempts are already being made to achieve this. A whole new industry has arisen around the need for predictive patch testing before new substances or cosmetics are let out into the community. Similarly, chrome allergy is less of a problem now in enlightened countries that insist on adding ferrous sulphate to cement to reduce its water-soluble chromate content. However, contact allergens will never be abolished completely and family doctors still need to know about the most common ones and where to find them (Table 7.3). It is not possible to guess which substances are likely to sensitize just by looking at their formulae. In fact, most allergens are relatively simple chemicals that have to bind to protein to become ‘complete’ anti-gens. Their ability to sensitize variesafrom substances that can do so after a single exposure (e.g. poison ivy), to those that need prolonged exposure (e.g. chromea bricklayers take an average of 10 years to become allergic to it).
The original site of the eruption gives a clue to the likely allergen but secondary spread may later obscure this. Easily recognizable patterns exist. Nickel allergy, for example, gives rise to eczema under jewellery, braclips and jean studs (Fig. 7.9).
The lax skin of the eyelids and genitalia is especially likely to become oedematous. Possible allergens are numerous and to spot the less common ones in the environment needs specialist knowledge. Table 7.3 lists some common allergens and their distribution.
Allergic contact dermatitis should be suspected if:
1 certain areas are involved, e.g. the eyelids, external auditory meati, hands (Fig. 7.10) or feet, and around gravitational ulcers;
2 there is known contact with the allergens mentionedin Table 7.3; or
3 the individual’s work carries a high risk, e.g. hair-dressing, working in a flower shop, or dentistry.
Questioning should cover both occupational and domestic exposure to allergens. The indications for patch testing have already been discussed. Techniques are constantly improving and derma-tologists will have access to a battery of common allergens, suitably diluted in a bland vehicle. These are applied in aluminium cups held in position on the skin for 2 or 3 days by tape. Patch testing will often start with a standard series (battery) of allergens whose selection is based on local experience. Table 7.3 shows the battery we use and how it helps us with the most common types of contact allergy. This picks up some 80% of reactions. Extra series of relevant allergens will be used for problems such as hand eczema, leg ulcers and suspected cosmetic allergy, and for those in jobs like dentistry or hairdressing, which carry unusual risks. Some allergies are more common than others: in most centres, nickel tops the list, with a positive reaction in some 15% of those tested; fragrance allergy usually comes second. It is import-ant to remember that positive reactions are not neces-sarily relevant to the patient’s current skin problem: some are simply ‘immunological scars’ left behind by previous unrelated problems.
Topical corticosteroids give temporary relief, but far more important is avoidance of the relevant allergen. Reducing exposure is usually not enough: active steps have to be taken to avoid the allergen completely. Job changes are sometimes needed to achieve this. Even then, other factors may come into play; e.g. some believe that reactions to nickel can be kept going by nickel in the diet, released from cans or steel saucepans, as changes in diet and cooking utensils may rarely be helpful.
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