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.




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.


Symptoms of allergies

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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Allergy Tests

The type of test to be carried out will depend upon your symptoms or condition of your skin and are described as follow:


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


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.



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