what is meant by radioallergosorbent test,principle involved in it.this test is used in immunology

Radioallergosorbent test:
An  href="http://www.medterms.com/script/main/art.asp?articlekey=6748">allergy
test done on a sample of blood. The test is used to check for allergic  href="http://www.medterms.com/script/main/art.asp?articlekey=24599">sensitivity
to specific substances. In the test, the sample of blood is mixed with substances known
to trigger allergies. The test measures the level of allergy antibodies (specific  href="http://www.medterms.com/script/main/art.asp?articlekey=21070">IgE
antibodies) in the blood which are present if there is a  href="http://www.medterms.com/script/main/art.asp?articlekey=2196">allergic
reaction. Because Radioallergosorbent test is a mouthful, it is best known
as  href="http://www.medterms.com/script/main/art.asp?articlekey=5210">RAST.

The
advisory panel on allergy, Internal medicine and otolaryngology believed that
radioallergosorbent test (RAST) is a recognised and effective test of
allergy.

RAST detect specific IgE antibodies responsible
for hypersensitivity: the allergen is bound to insoluble material and the patient's
serum is reacted with this conjugate; if the serum contains antibody to the allergen, it
will be complexed to the allergen. Radiolabeled anti-human IgE antibody is added where
it reacts with the bound IgE. The amount of radioactivity is proportional to the serum
IgE.

RAST tests
are often used in combination with skin tests, or in situations when other tests are
considered risky (e.g., when a patient has experienced a severe allergic reaction after
eating a food). Because the test takes place in blood drawn from the patient, and not in
the patient's skin or body, there is no risk of adverse reaction as with a  href="http://foodallergies.about.com/od/diagnosingfoodallergies/p/pricktests.htm">prick
test or a food
challenge.

Laboratory
technicians add the likely  href="http://foodallergies.about.com/od/glossary/g/allergen.htm">allergen,
bonded to a solid polymer, to the patient's blood. While there may be a large quantity
of immunoglobulin
E (IgE) -- the major antibody that binds to allergens in allergic reactions --
in the blood, the type that binds to each allergen is slightly different, and is
referred to as allergen-specific
IgE.

If the patient is allergic to the allergen being
tested for, allergen-specific IgE will attach to the allergen and other IgE (from
anything else the patient is allergic to) will float freely in the blood. The blood is
then "washed," leaving the allergen and any of the patient's attached allergen-specific
IgE.

A serum of radioactive anti-IgE, which is derived from
people who are known to be allergic to the allergen being tested for, is then added,
allowing technicians to determine the concentration of allergen-specific IgE in the
patient's blood.