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Reviewed by Dr Victoria Lewis, specialist registrar in dermatology
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Delayed hypersensitivity
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| Nickel is a common cause of allergic contact
eczema. |
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The cells of the
immune system work in various ways in
skin disease.
The rash that occurs after being stung by nettles happens within
minutes of contact. It is due to the release of chemicals such as histamine by
immunity cells within the skin layers. This is known as ‘immediate
hypersensitivity’.
Slower acting forms of response by the immune system occur
because the population of cells that react against the invading substance or
organism (allergen) needs to be built up from a small number of ‘memory’ cells
that recognise the allergen.
Over two or three days these cells give rise to others that in
turn switch on other defence mechanisms such as the release of chemicals that
attract the scavenger cells of the immune system. Because of the lag between
initial contact and subsequent reaction, this is called ‘delayed
hypersensitivity’. The skin reactions underlying allergic contact eczema are of
this type.
Common allergens
It is not known why some substances are particularly prone to
causing contact allergy, but common culprits are:
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nickel - a white metal often used in jewellery, belt buckles,
jean studs, metal watch straps, bra fasteners. Many other metals can also cause
reactions.
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perfumes - Balsam of Peru is a component of fragrances that can
cause contact allergy.
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rubber and related chemicals - shoes, tyres and
clothing.
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potassium dichromate - cement, leather and matches.
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hairdressing chemicals (paraphenylenediamine).
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medications - lanolin, neomycin (antibiotic), fusidic acid
(antibiotic), steroid creams and cetearyl alcohol (used in emollient
creams).
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epoxy resins - in glues used in hobbies, woodworking and glass
fibre construction.
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colophony - resin from spruce trees, used in sticking plaster
adhesive.
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plants - both by direct contact and by contact with airborne
particles.
In the food industry a wide variety of foods are known to
potentially cause skin reactions. These include:
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sugar
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flour/dough
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citrus fruits and their peel
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other fruits and vegetables
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spices, herbs, seasonings (eg horseradish, mustard, garlic)
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fish and seafoods
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meat and poultry.
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 (eg lanolin in
moisturisers and hydrocortisone in
steroid creams). 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.
Features of contact allergic eczema
The rash first appears only at the site of skin contact with
the allergen. Later it often spreads out and may involve skin well away from
the contact site. This is because the immune cells become activated and migrate
out through the bloodstream to other regions.
If allergic contact eczema is suspected, it can therefore be
important to remember where it first started when taking the medical history
behind the development of a rash.
Sometimes the cause and effect relationship is obvious, with a
red scaly reaction around an earring or under a watch strap. At other times,
especially if a long time has passed since the initial exposure and
sensitisation, it may be impossible to distinguish contact allergic from other
types of eczema.
There can also be difficulties distinguishing allergic contact
eczema (which is an immune response) from irritant eczema (which is a direct
skin reaction against an irritant substance). In practice, this difference is
not so important because the same lines of treatment are required for both.
In particular, a search needs to be made to try and identify a
possible allergen. The most common way to do this is to test a person’s skin
against a battery of standard test substances, all of which are known to
possibly cause contact allergy. This is called 'patch
testing’.
Treatment
If
allergy testing reveals a cause that
can be removed, the problem is quite likely to resolve. This is not true for
everyone, particularly individuals with hand eczema. It is unclear why this is
so. Sensitivity will remain dormant, however, and later exposure will be
followed by eczema reaction.
Moisturisers and
steroid creams do work for contact
eczema, but the extent of improvement depends also on the degree of
sensitisation and the level and frequency of exposure to the allergen.
If the offending substance is repeatedly being re-applied to the
skin, you can end up using a lot of treatment to little effect.
The new
cytokine inhibitor creams such as
pimecrolimus and tacrolimus have not been around long enough for their effect
to be judged in contact eczema. It is likely they will be as effective as
steroids, if not more so.
Other types of eczema
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Based on a text by Dr Dan Rutherford, GP
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Last updated 15.09.2005
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