Testing For Allergies 223The only suitable indications for the use of this method are in
cases of perennial rhinitis and asthma due to dust and dust mite allergy,
seasonal rhinitis due to pollens and the danger of anaphylaxis due to insect
stings. Even so, very brittle (vulnerable) cases, especially children, are better
left un-desensitized, since the dangers of the method are very real.
Immunological tests
Other conventional allergy tests are based on immunological
reactions of the IgE antibody type, or occasionally IgA and IgG. Naturally,
classic antigen-antibody reactions are the only results to be expected, which
precludes a large number of other allergic and intolerant reactions.
Broadly speaking, then these tests are fine if they give a positive
result; which is to say, if an allergy to wheat or egg is found, it probably
exists. Avoiding that food will help with the overall body load. But negatives
are meaningless (these occur over 95 per cent of the time when foods
are being tested). Also, a positive reaction on an immunological test only
means antibodies are present, it does not mean the allergy is the one causing
symptoms. Experts often overlook this elementary point.
The RAST (Radio-Allergo Sorbent) Test
This is the basic immunological test using labeled molecules for
quantifying results.
Specific antigen (in this case the allergy being tested) is incubated
in contact with a plastic plate or tube until it becomes bound by adsorption
(adhesion).
The plate is washed and the patient’s serum, with suspected IgE
antibody to that allergy, is added. This naturally locks on to the antigen.
Unused sample fluid is washed away, leaving “captured” IgE.
How much IgE antibody has been captured is measured by adding
a second binding antigen, in this case radio-active anti-IgE, which locks
onto the IgE in the antigen-antibody complex. This creates an antigen-IgE-
antigen “sandwich”.
After unused labeled molecules have been washed away, the
amount that remains behind can be measured using a gamma counter
(Geiger counter). Hence: radio-allergo sorbent test or RAST.