Allergies

What is being analysed?
Immunoglobulin E (IgE) is a protein associated with allergic reactions; it is normally found in the blood at very low levels. IgE is produced as needed; it is part of the immune system and defends the body against “foreign” substances. It is produced when a genetically predisposed person is first exposed to a potential antigen (a food, grass or animal dander, something that causes no reaction in people without allergies). The body perceives the potential allergen as a threat and produces specific IgE proteins. These IgE antibodies bind to mast cells (specialised cells found in the tissues) and basophils (a type of white blood cell). Mast cells are found in high concentrations in the skin and in the respiratory and gastrointestinal tracts. On subsequent exposure, the IgE antibodies bound to the cells recognise the antigen and cause mast cells and basophils to release histamine and other substances, triggering an allergic reaction in the tissue where the exposure occurred.

The allergen-specific IgE test is used to detect an allergy to a specific allergen by measuring its level in the blood. Each allergen requires a specific test, and the tests are very specific, distinguishing, for example, between bee and wasp, egg white and yolk, or different species of ragweed. Allergens can be grouped into food, weed and grass, and mould panels; otherwise each suspected allergen must be specified.

Several methods are available to measure allergen-specific IgE. The longest used and most studied is the RAST (radioallergosorbent test). Some doctors call all IgE methods RAST, even though RAST is a specific radioimmunoassay method.

How is the sample collected for testing?
The blood sample is taken from a vein.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
Allergen-specific IgE testing is used to confirm an allergy (type I hypersensitivity) to one or more specific allergens after acute or chronic allergy-like symptoms have appeared.

Allergen-specific IgE testing may be carried out (as an alternative to other allergy tests) in the case of significant eczema or dermatitis, or treatment with antihistamines or antidepressants that could affect other tests, or to avoid a potentially dangerous allergic reaction.

Allergen-specific IgE testing, although it can only be used in a general way, may be carried out to monitor immunotherapy or to check whether a child has outgrown an allergy; IgE levels do not correlate with the severity of the allergic reaction and can remain high for many years even after the allergy has gone.
 
When is it requested?
Allergen-specific IgE testing is usually requested when there are signs or symptoms suggestive of an allergy to one or more substances or foods.

What does the test result mean?
Negative results very probably rule out a “true allergy”, i.e. an IgE-mediated response to a specific allergen, but they must be interpreted and used with caution and with your doctor’s advice. Even if the IgE test is negative, there is still a small chance of allergy.

High levels usually indicate an allergy, but even if specific IgE is positive, you may or may not have an allergic reaction after exposure to the allergen. The amount of IgE does not necessarily predict how severe a reaction could be. To confirm a diagnosis of allergy, the medical history and other allergy tests under close medical supervision are needed.

Is there anything else I should know?
Other blood tests may be needed to follow an allergic process, such as total IgE and a full blood count with white cell differential (paying particular attention to basophils and eosinophils). A rise in these may suggest an allergy but can also have other causes.

1. What other tests are available to assess an allergy?
2. If the test is negative but I still have symptoms, what else can I do?
3. My allergy symptoms are moderate. How serious is my allergy really?
4. Will I ever get over my allergy?
5. Why am I told to avoid fresh fruit when I am allergic to pollen?

1. What other tests are available to assess an allergy?
The skin prick or scratch test, the patch test and the oral food challenge carried out by an allergist or dermatologist. You can also try eliminating foods from your diet and then reintroducing them to find out which one you are allergic to. It is important that these tests are done under close medical supervision, because an anaphylactic reaction can be life-threatening.

2. If the test is negative but I still have symptoms, what else can I do?
There may be a genetic hypersensitivity, such as gluten sensitivity in coeliac disease, or an enzyme deficiency such as lactase deficiency, which causes lactose intolerance. It could also be an allergic condition not mediated by IgE, for which no specific laboratory tests are available, or another disease with allergy-like symptoms. Each case needs to be investigated individually.

3. My allergy symptoms are moderate. How serious is my allergy really?
Allergic reactions are very individual. They can be moderate or severe, vary from one exposure to the next, get worse (or not) over time, involve the whole body and sometimes be fatal.

4. Will I ever get over my allergy?
Although children can outgrow some allergies, adults do not. The allergies that cause the worst reactions, such as anaphylaxis to peanuts, do not go away. The best remedy is to avoid exposure to the allergen and to be prepared in advance for accidental exposure with medicines such as antihistamines and ready-to-use adrenaline injections. Immunotherapy can reduce the symptoms of some allergies to allergens that cannot be avoided, but it is not effective for foods, and treatment, which usually consists of years of regular injections, may need to continue indefinitely.

5. Why am I told to avoid fresh fruit when I am allergic to pollen?
There is cross-reactivity between some inhaled allergens and fruit proteins. The body thinks it recognises the pollen and causes an allergic reaction to the fruit. This is, however, rare.