ALLERGY TESTING: CONVENTIONAL ALLERGY TESTS
Prick Test
Extracts of well-known allergens such as pollen, house mite or cat dander, are manufactured by drug companies for use in a variety of tests, to ascertain which substances are allergenic to a specific individual.
A simple test is carried out, by placing a drop of the extract on the skin of the forearm and gently pricking it into the upper layer of skin. Up to twenty, or more, of these pricks, each with a different substance, can be carried out in one visit to the allergist. If you are sensitive to a substance, the spot will swell slightly and become red within about fifteen minutes. This does not mean, however, that you are allergic to that substance, simply that, were you exposed to it often enough, or for long enough, you might become allergic to it. It could also mean that you were once allergic to it but, for some reason, are no longer. Even a positive result is far from conclusive and, at best, gives a vague indication only. Nevertheless, this form of testing has had some success in diagnosis and treatment of the airborne, or household, type of allergy. It is also quite often used to test for food allergies and, for this purpose, it is totally unreliable.
Because allergies affect the immune system, it is possible, by examining its structure, to see why skin tests do not detect food and chemical allergies. In Chapter 3 mention was made of antibodies which the immune system manufactures in order to destroy antigens. These antibodies are known medically as 'immunoglobulins', of which there are five main types, IgE, IgD, IgG, IgM and IgA.
IgE is bound to mast cells and resides mainly in the skin mucosa, where it is responsible for contact reactions and allergic manifestations, such as urticaria. In the case of an allergy reaction, the IgE, when confronted with an antigen such as pollen, does not destroy it. Instead, the antigen causes the mast cell, to which the IgE is attached, to break up and release a flood of histamine, which in turn gives rise to the allergy symptoms. IgD is thought to initiate IgE production. IgG and IgM antibodies are found mainly in the blood. It is their job to travel the body and ward off infection by dealing swiftly with invading antigens such as foreign bacteria. In addition IgM acts as a rallying point for a number of IgGs to bind together and form a stronger defence mechanism.
IgA is found mainly in the gut and is produced by the gastrointestinal cells. It acts in a similar way to IgG and IgM, but, instead of patrolling the body, it remains in situ in the stomach and intestine. There, it is ready to attack harmful substances brought in to the body by the process of eating and drinking.
Skin testing, therefore, is likely to stimulate only the IgE system. The systems most likely to be involved with food and ingested chemicals, namely the IgG, IgM and IgA systems, cannot be reached through skin testing. Consequently, skin testing can stimulate the IgE system to produce an allergic response to pollens and house mites, but will fail to stimulate other antibodies to react to food substances.
Patch Test
This is a similar test to the skin prick test except it is used only to ascertain allergies to the skin. In this respect, it is a sensible exercise and has a reasonable success rate. It is effected by a piece of the allergenic substance being taped, or placed, onto the skin for a period of twenty-four hours. Subsequent reaction will then indicate whether an allergy to the substance exists.
Intradermal Test
This is really an extension of the skin prick test. Instead of pricking the substance into the skin, it is injected into the outer layers of skin, on the outside of the upper arm or the inside of the forearm. It is felt that in some cases, this method may give a better response than the prick test, however, both tests are very similar in their application. If the reaction is positive, a small red swelling will show within ten minutes. Sometimes, however, a delayed reaction may occur several hours later.
The intradermal skin test stimulates the IgE system, found mainly in the skin, to react to allergens which normally enter the body through the skin. It relies upon histamine, produced by the IgE mast cells, to produce the red swelling which indicates an allergic response. Clinical ecologists have found that IgE is often not present with IgG, IgM and IgA in food allergies and, therefore, a skin test,
which is IgE mediated, used to isolate an immune disturbance that is IgG or IgM mediated, will give an inaccurate result. Skin testing, therefore is of limited use and it is notoriously inaccurate for food testing but it can help to detect some allergies in certain people.
Sublingual Test
The test substance, in solution, is placed under the tongue. This test is sometimes used by doctors in an attempt to identify suspect food allergies. It is not effective for similar reasons to those that render skin tests ineffective. However, whereas the skin test is IgE mediated, the sublingual test is, probably, IgA mediated, whilst the food allergies being tested, are often IgG or IgM mediated. This is another example of testing which does not contact the right immunoglobulin to enable an accurate result to be achieved.
Cytotoxic Testing
The word 'cytotoxic' is derived from the Greek word CYTO which means 'cells' and 'toxic' which means 'poisonous'. A cytotoxic reaction, therefore, is one that effects the cells, or more specifically, the white blood cells.
It is now possible to observe the effect some food substances have on the white blood cells, under a microscope. This is due to work done by an American, Dr Arthur Black, in the 1950s. In Australia, there are about a hundred foods that can be tested in this manner which involves taking a sample of blood and submitting it for laboratory analysis.
The test is unique because, as not all allergic reactions are mediated by antibodies, ecological problems, in the form of non-immunological allergies, can sometimes be detected. This method can also discover reactions to substances that, otherwise, produce no symptoms — that is, none that are yet detectable by the individual — even though they may already be damaging the body. This application can be useful in the detection of masked food allergies, which often provoke stray cravings, without identifiable aftereffects. Some chemical intolerances can also be detected with this method.
Unfortunately, cytotoxic food testing, like most other allergy tests, does not always produce accurate, or complete, results. It requires only one major allergen to remain undiscovered, for the ongoing and distressing effects of allergy illness to continue.
RAST Test (Radio-allergo sorbant assay)
This is a fairly recent immunological test which is not solely reliant on the response of only one immunoglobulin, such as with the skin test. The antigen to be tested is fixed to small particles and labelled by a radioactive process. It is then mixed with a specimen of the individual's blood serum in order to stimulate a response from one of the immunoglobulins. The presence of immunoglobulins, other than IgE, provides scope for a more accurate allergic response to the antigen. The blood serum is washed and radioactively counted to find out which immunoglobulin has reacted with the antigen, thus indicating whether an allergic reaction has taken place.
Although this test is used extensively in the United States, it is not freely available elsewhere. It is certainly an improvement on skin testing as it has been found to be useful in detecting food and chemical allergies. However, it does not always produce accurate answers and so is of limited use compared with other forms of testing which are now available in Britain and America.
*65\106\2*
Allergies
