Rast

Immunoglobulin E (IgE) is a protein involved with allergic reactions. It is normally found in very small amounts in the blood and attached to the surface of certain specialised cells (mast cells and basophils), that are found throughout the body. IgE is a type of antibody and part of your immune system. It helps to defend the body against harmful intruders. However, sometimes an individual’s immune system will make IgE to harmless substances such as pollen, a particular food or animal dander (dead skin cells shed by all furry and feathered animals). We don’t fully understand why the immune system of some people responds like this but when it does we refer to these harmless substances as allergens. When someone with a predisposition to develop allergy is exposed to one of these allergens, they may produce IgE that is able to bind to the allergen. Their body sees the potential allergen as a foreign substance and produces IgE that binds to the allergen and attached to the surface of mast cells and basophils. The IgE produced will only bind the particular allergen involved and is known as an allergen specific IgE. For example someone may respond to eating a peanut by producing “peanut specific IgE”.

Mast cells are particularly concentrated in the skin, respiratory system, and gastrointestinal tract, while basophils are found mainly in the blood. IgE attaches to the surface of these cells and then waits for an opportunity to meet the allergen they were made to bind. The next time that you have contact with the allergen concerned, it will recognised by the IgE on the surface of these cells. The IgE then send signals to the cell to cause it to react to the allergen. The mast cell or basophil then releases histamine and other chemicals which cause the symptoms of an allergic reaction.

The allergen specific IgE test measures the blood IgE antibody level for a particular allergen, e.g. peanut, birch tree pollen, grass pollen.

Each test measures the IgE level for a very particular allergen. For example, honeybee, bumblebee and wasp are three different tests. Sometimes tests are performed as panels when it isn’t clear exactly which substance should be tested for, e.g. tree pollens may be tested as a group rather than doing individual tests for every possible tree pollen. Your doctor can choose from a long list of individual allergens - there are several hundred different allergen specific IgE tests available. It’s therefore very important to consider the history of your symptoms so that the correct tests can be performed and to reduce unnecessary testing for substances that you are unlike to be allergic to.

The allergen-specific IgE test can be done using a variety of methods. The traditional method is the RAST (radioallergosorbent test) but it has been largely replaced in most laboratories with the newer methods. RAST is still a convenient shorthand term in common use and many doctors still refer to all IgE allergy tests as RAST even though this may not be the exact method that the testing lab is using.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.