Allergy and allergy tests

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What is allergy?

Allergy is when the immune system reacts to a substance that is eaten (e.g. food, medication), inhaled (e.g. pollen, house dust mite) or injected (e.g. drug, insect sting).

These substances are called allergens.

What types of things can cause allergic reactions?

Common things that people are allergic to include food (e.g. milk, eggs, wheat, soy, peanuts and tree nuts, fish and shellfish, ), pollens, grasses, house dust mite and animals or insect stings.

Are there different types of allergy?

Allergic reactions are often divided on the basis of how  quickly they occur: 

Immediate type reactions

  • Usually happen within  2 hours of a person coming into contact with whatever they are allergic to .
  • This type of reaction can be associated with anaphylaxis.
  • These reactions are caused by a reaction between a protein in the blood called Immunoglobulin E (IgE) and the substance you are allergic to. 
  • The allergen can often be confirmed using skin or blood tests.

Delayed type reactions

  • These  occur after a longer period, often more than  24 hours later .
  • They are not associated with anaphylaxis
  • Delayed reactions do not usually involve IgE and cannot be diagnosed on skin and blood tests.

What types of allergic reactions can you get?

Allergy to pollens, house dust mite and animal dander usually causes symptoms of allergic rhinitis (runny nose, blocked nose, itchy eyes). They may also cause symptoms (wheeze and cough) in people with asthma. When caused by seasonal pollens, it is called hay fever. If it goes on throughout the year then the allergen is usually one that persists year-around (e.g. dute mite, animal dander).

Immediate allergy to foods may cause skin problems such as hives and itch or bowel problems such as tummy pain, vomiting or diarrhoea. Occasionally, people may develop problems with their breathing passages or lungs e.g. swelling of the tongue, cough, hoarse voice or wheeze.  This is called anaphylaxis.

In some people with eczema, their eczema may get worse if they eat a food they are allergic to.  This is a delayed type reaction and can’t be tested for with skin or blood tests.

What is anaphylaxis?

Anaphylaxis is a severe allergic reaction.

It involves breathing problems and sometimes a low blood pressure or even collapse and unconsciousness which can be life threatening.

It requires immediate life-saving medication

What is a skin test?

There are different types of skin tests. The most common type of skin test to investigate allergies is a skin prick test.

In a skin prick test, a small drop of a protein extract (allergen) is placed on the skin (usually the forearm) and a small prick is made in the skin through the drop. The size of the swelling (wheal) is measured after 10- 15 minutes.

What is a positive skin test?

A skin prick test which results in a wheal (hive) bigger than 3mm by3mm is positive.

Skin tests are useful for detecting immediate type allergies; they do not detect delayed reactions.

My child’s skin test was positive, what does that mean?

Allergy test results cannot be used on their own. They must be judged together with a medical history.

If they have hay fever symptoms and have a positive skin test for a protein commonly linked with hay fever (e.g. pollen), then their symptoms may be due to that protein.

Children with allergic asthma may have a positive result for the substance that causes many of their asthma symptoms.

If your child has had an allergic reaction recently (e.g. hives) when they ate a certain food, a positive skin may help confirm that they are allergic to that food.

A positive skin test for a food that your child has never, or at least not recently, eaten means that they are sensitised to that food. This means that their body has produced IgE antibody against that food protein. It means they could have an allergic reaction if they eat the food.

Many people will have a positive skin test for foods they already eat with no problems and should not stop eating those foods unless directed by their doctor.

As well as helping to diagnose an allergy if they are positive, skin tests are very useful if they are negative. If a skin test is negative, it is extremely unlikely that your child has an immediate allergy to that food.

Why are skin tests done?

If your child has had a recent allergic reaction after eating but you are not sure what caused the reaction, a skin test might (but does not always) help to identify the cause.

Skin tests can be used to monitor allergies i.e. if the skin test becomes smaller as your child gets older they are probably no longer allergic to the food and may need a food challenge.

The size of the reaction (wheal) is not helpful in predicting the risk of anaphylaxis.

What is a blood test?

A RAST test (also called a specific IgE test) measures the IgE antibodies in the blood for a specific allergen.  It is used when skin testing cannot be performed, e.g. because of severe eczema or a person taking regular anti-histamines.

So how do you determine if the foods that came up positive on a skin test will be a problem?

  • If your child has also recently had a definite immediate allergic reaction after eating a food then a positive skin test can confirm that allergy.
  • When they have not had a definite reaction to a food it can be difficult to decide if they are really allergic.  Reasons for this include, ,
    • Often symptoms are vague and don’t obviously happen immediately after eating the suspected food. This is particularly true of bowel symptoms like pain or diarrhoea.
    • Occasionally your child has never eaten the food for which their skin test is positive or it has been a long time since they last had a reaction or contact with the suspected food.
    • Sometimes you are not sure whether they actually ate the food or if it simply came into contact with the skin.
  • In these circumstances, the only sure way to diagnose food allergy is with a food challenge.

Why is it so important to find out for sure whether my child is really allergic? Can’t I just assume my child will have a reaction to the foods that are positive on their skin tests?

  • Allergies, especially food allergies are not simply a nuisance; they do have a large impact on many aspects of life.
  • There are a few very important reasons to find out the true nature of suspected allergies:
    • If your child is really allergic, you need to know whether the allergy is potentially life so that appropriate precautions can be taken.
    • Your child may not be allergic! It is a great pity to be labeled as allergic to a food that actually causes no problems. This may lead to significant anxiety, restrictions in where they can eat and in some cases children have been placed on such restrictive diets that they are malnourished.
    • There is some evidence that avoiding foods to which the child is not already allergic may actually predispose to allergy. 
    • The diagnosis of allergy has money and lifestyle affects  


  • Most allergy is caused by a reaction between IgE and another substance
  • Allergies may cause hay fever, asthma or reactions to foods such as hives
  • The most severe type of allergy is called anaphylaxis
  • A positive skin or blood test for foods does not necessarily mean that your child will have an allergic reaction if they eat the food and sometimes a challenge is needed to sort this problem out
Written by the Immunology Department, Sydney Children's Hospital
The Children's Hospital at Westmead
Sydney Children's Hospital, Randwick
Hunter New England Kids Health

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