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Allergies
What is Allergy?
What is an Allergen?
Symptoms of Allergy
What Allergy tests are available?
What is Allergy?
The term allergy is used to describe a response, within the
body, to a substance, which is not necessarily harmful in
itself, but results in an immune response and a reaction that
causes symptoms and disease in a predisposed person, which
in turn can cause inconvenience, or a great deal of misery.
Allergy is extremely widespread and affects approximately
one in four of the population. Each year the numbers are increasing
by 5% with as many as half of all sufferers being children.
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What Is An Allergen?
Almost anything can be an allergen; the most common being
house dust mites, pollen from trees and grasses, cats, dogs,
insects such as wasps and bees, milk, eggs, peanuts. Less
common allergens include nuts, fruit and latex. Protein is
often regarded as just something that we eat. It is, in fact,
an organic compound containing hydrogen, oxygen and nitrogen,
which form an important part of living organisms. There are,
however, some non-protein allergens which include penicillin
and some other drugs. For these to cause an allergic response
they need to be bound to a protein once they are in the body.
A person's immune system believes that the invading allergens
are damaging and in response the body produces an antibody
("IgE") to attack them. This in turn leads to other
cells (i.e. mast cells) to release further chemicals which
together cause irritation, inflammation and the symptoms of
an allergic response. Some of the conditions or diseases themselves
have already been listed but it should be understood that
all the mentioned diseases can be caused by factors other
than allergy.
Here are some of the most common symptoms associated with
the conditions:
- Sneezing
- Wheezing
- Sinus pain
- Runny nose
- Coughing
- Nettle rash / hives
- Swelling
- Itchy eyes, ears, lips throat
& palate (roof of mouth)
- Shortness of breath
- Sickness vomiting, & diarrhea
- Increase in secretions
Source: British Allergy Foundation
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Symptoms of allergy
A person's immune system believes that the invading allergens
are damaging and in response the body produces an antibody
("IgE") to attack them. This in turn leads to other
cells (i.e. mast cells) to release further chemicals which
together cause irritation, inflammation and the symptoms of
an allergic response. Some of the conditions
or diseases themselves have already been listed but it
should be understood that all the mentioned diseases can be
caused by factors other than allergy.
Here are some of the most common symptoms associated with
the conditions:
- Sneezing
- Swelling
- Wheezing
- Itchy eyes, ears, lips throat & palate (roof of mouth)
- Sinus pain
- Shortness of breath
- Runny nose
- Sickness vomiting, & diarrhoea
- Coughing
- Increase in secretions
- Nettle rash / hives
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What Allergy tests are available?
The type of test to be carried out will depend upon your
symptoms or condition of your skin and are described as follows.
Skin Prick Testing:
This is a test, which measures specific IgE attached to cells
in the skin important in allergies called "mast"
cells. This is probably the most commonly used allergy test
and is appropriate for inhaled and ingested (eaten) allergies:
- It is usually carried out on the inner forearm (palm uppermost)
- If the patient has bad eczema on the area under test then
the test can be performed on the back
- Ideally the allergens to be selected should be in accordance
with the patient's history
- As few as 3 or 4 or up to about 25 allergens can be tested
- The arm is coded with a marker pen for the allergens to
be tested
- A drop of the allergen (extract) solution is placed by
each code
- The skin is then pricked through the drop using the tip
of a lancet -this can feel a little uncomfortable but should
not be painful
Positive reaction:
The skin becomes itchy within a few minutes and then becomes
red and swollen with a "weal" in the centre (very
much like the reaction to a nettle sting). The weal has a
raised edge, which slowly expands to reach its maximum size
in about 15-20 minutes, clearing for most people within an
hour. The size of the weal varies with the average being 3-5
mm in diameter.
Also included in skin testing is a negative and positive
control:
The negative control is a saline (salt-water) solution, to
which a response is not expected. If however a patient reacts
to a negative control, then this will indicate that the skin
is, for what ever reason, extremely sensitive and that the
results from the allergen challenge needs to be interpreted
with the utmost care.
The positive control solution contains histamine, to which
everyone is expected to react. Failure to do so could mean
that medicines the sufferer is taking could block the response
to the histamine and allergens. Patients will be asked to
avoid taking anti histamines, cough medicine and some anti
depressants (Tricyclic) for about 5-6 days prior to the test.
The skin prick test introduces such a tiny amount of allergen
into the skin that testing is quite safe and almost any age
group can be tested. However where there has been a clear
anaphylactic (shock) reaction to a specific allergen ingested
then skin testing may not be appropriate. These tests can
be carried out on all age groups including babies although
the response will be considerably smaller than in an adult.
Skin prick testing is usually the first test recommended
when an allergy is suspected. The advantages are that it is
a simple, quick and inexpensive form of testing. It can give
useful information in all forms of allergy and provides results
within 15-20 minutes. This can be carried out within a hospital
or GP surgery environment, by specially trained nurses or
doctors.
Blood Test:
The test is carried out on a small sample of blood, which
is usually taken from a vein in the arm, using a fine needle
and a small syringe, causing minimal discomfort. The sample
is then sent to a hospital laboratory and the results are
available in 7 to 14 days. The blood sample can be taken at
the GP surgery or at a hospital.
These tests are particularly useful when:
- The patient has a risk of an anaphalactic (shock) reaction,
which would make skin prick testing too risky
- When extensive eczema makes skin prick testing impractical
- When antihistamine medication cannot be stopped because
of severe symptoms
- No skin prick testing facilities are available
Where unusual and rare allergens are suspected, as there
are a wide range of UniCAP® allergens now available for
testing
Positive Test: - UniCAP is reported in classes of 0-6 where
increasing classes indicate increasing sensitivity to an allergen:
Class 0 Negative
1 Low
2 Moderate
3 High
4 -6 Extremely High
Patch Testing:
This test is performed in cases of contact dermatitis (Eczema)
where allergy is suspected.
- The allergens are prepared in appropriate concentrations
in white soft paraffin (e.g. Vaseline) and are then spread
on to discs, 1 cm diameter.
- The discs (which are made of a special metal, cannot themselves
provoke a reaction) are placed on the skin, usually on the
back, and are kept in place by hypoallergenic tape.
- The skin is coded appropriately and the patient is asked
to keep the skin dry.
- The patches are left in place for 48 hours, after which
time the discs are removed, the skin is examined and any
redness or swellings are noted. The skin is re-examined
after a further 48 hours for any remaining local redness
or swelling.
The interpretation of this form of testing is not as simple
as it sounds and tends to be carried out by dermatology departments
in hospitals. The symptoms of contact dermatitis need to be
brought under control before patch testing can be carried
out; otherwise the results will be unreliable. Steroid creams
need to be stopped for 3-4 weeks before testing as they may
suppress the test response.
Any professional interpreting skin, blood or patch tests
must first interpret the results in the light of the patient's
history. No test should be read in isolation.
Source: The British Allergy Foundation
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