A blood sample taken from a vein in your arm
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.
How is it used?
The allergen-specific IgE antibody test is done to test for an allergy (a type I hypersensitivity) to a specific substance or substances when a patient presents with acute or chronic allergy-like symptoms.
The allergen-specific IgE antibody test may be done when skin prick tests are not possible. If the patient has significant dermatitis or eczema or is taking anti-histamines, skin prick tests are difficult to perform and/or interpret.
The allergen-specific IgE antibody test has limited use in checking whether a patient has outgrown an allergy as the test may remain positive even in someone who has long outgrown their allergy. The usefulness of allergen-specific IgE to monitor response to immunotherapy is unclear, and may remain positive even after successful immunotherapy.
When is it requested?
What does the test result mean?
Normal (negative) results indicate that you probably do not have a "true allergy", an IgE-mediated response to that specific allergen, but the results of allergen-specific IgE antibody tests must always be interpreted and used with caution and with the advice of your doctor. No test is perfect and, though a negative test means allergy to that substance is unlikely, it does not completely exclude allergy.
Elevated results often indicate an allergy. However, not all patients with a positive specific IgE test will have an actual physical allergic reaction when exposed to that substance – the patient in this case is said to be “sensitised” rather than allergic to the substance. The amount of specific IgE present does not necessarily predict the severity of a reaction, although the higher the level the more likely the patient is to have a reaction of some sort to the allergen. Your clinical history and other allergy tests, done under close medical supervision, may be necessary to confirm an allergy diagnosis.
Is there anything else I should know?
Sometimes your doctor will look at other blood tests for an indirect indication of an ongoing allergic process, including your total IgE level or your full blood count (FBC) and white blood cell differential (particularly eosinophils). Elevations in these tests may suggest an allergy, but they may also be elevated for other reasons.
What other tests are available for allergy testing?
Skin prick or intradermal tests, and oral food or drug challenges are usually done by an allergist or immunologist. Your doctor may also try eliminating foods from your diet and may reintroduce them to find out what you are allergic to. It is important that these tests be done under close medical supervision, as a life threatening anaphylactic reaction, though rare, is possible.
Patch tests are used in the investigation of 'allergic contact dermatitis', and check for delayed reactions to substances which are in contact with the skin, eg fragrance in cosmetics, nickel. These tests are may be performed by immunologists, allergists and dermatologists. These reactions do not depend on IgE and so the IgE test may be normal. However, these generally have very different signs and symptoms to IgE related disease. A carful assessment of your symptoms will enable your doctor to choose the appropriate tests for you.
My allergy test was negative, but I am having symptoms. What else could it be?
You could have an allergy-like condition or intolerance that is not caused by IgE for which there are no specific laboratory tests. Or it could be another disease that is causing allergy-like symptoms. It is important to investigate your individual situation with your doctor's assistance. In the case of bowel symptoms after food, there can be other causes such as coeliac disease, an autoimmune reaction to gluten, or lactose intolerance, caused by a common enzyme deficiency.
My allergy symptoms are generally mild. How serious is this really?
The severity of an allergic reaction varies greatly from one person to another. They can be mild or severe, vary from exposure to exposure, get better or worse or not change over time. They may be local or involve the whole body. Although most people cope with their allergy, fatalities do rarely occur, mainly in patients with severe food allergy.
Will my allergies ever go away?
Although children do outgrow some allergies, adults usually do not. Allergies that cause the worst reactions, such as anaphylaxis caused by peanuts, do not usually go away. Avoidance of the allergen and advance preparation for accidental exposure, in the form of drugs such as antihistamines and portable adrenaline injections, is the safest course. Immunotherapy can help decrease symptoms for some unavoidable aeroallergens such as pollens, and this treatment usually consists of years of regular injections. This approach is not advocated for food allergens.
Why am I told to avoid fresh fruit when my allergy is to tree pollen?
This is due to “oral allergy syndrome”. There are substances in tree pollen that are very similar to those in certain fresh fruits. If your immune system makes IgE to these substances in pollen, this IgE may also “crossreact” with the fruit proteins. Your body thinks it is detecting and reacting to tree pollen when in fact it is fruit that it is responding to. Oral allergy syndrome is quite becoming more common, though in most cases it is mild, with tingling or a scratch sensation in the mouth or throat.