OTHER FORMS OF FOOD ALLERGY: THE RADIOALLERGOSORBENT TEST (RAST)
The radioallergosorbent test, or RAST, measures the level of IgE antibodies that a person has to a specific substance, such as a food protein or a pollen. There are four stages to the test:
1. An extract of the food (or other potential allergen) is applied to beads made of a substance called sepharose. This is an inert substance that simply acts as a surface on which reactions between the allergen and the antibody can take place. The food molecules remain attached to the sepharose beads throughout the test.
2. A sample of the patient’s serum (the liquid part of the blood) is allowed to flow over the beads. If the blood contains IgE antibodies to that food, these will bind to the food antigens on the beads. The beads are later rinsed to remove everything that is not bound – only the IgE molecules should remain.
3. Another liquid is poured over the beads. This contains a special type of antibody called anti-lgE, which binds specifically to the stem of IgE molecules. If there is IgE stuck to the beads, these anti-lgE antibodies will bind to them. If no IgE is present, then all the anti-lgE will be washed away.
4. The anti-lgE was previously marked with a radioactive marker or a coloured marker. This means that the amount of IgE present can be worked out by measuring the radioactivity or colour given off by the beads. The amount of anti-lgE present is a measure of how much IgE (specific for that food) there is in the patient’s blood.
Of course, the test will also give a positive result if the food contains something that specifically binds to IgE (as long as there is some IgE in the blood). This does indeed happen, in some patients with false food allergy.
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