IgE mediated allergies
A classic example of an immunoglobulin E or IgE mediated allergy is peanut allergy.
When someone has a peanut allergy, their immune system mistakenly treats proteins in peanuts as abnormal. The first time a person with a peanut allergy is exposed to peanuts, their immune system creates specific antibodies called IgE antibodies. This process is known as “sensitisation.”
These IgE antibodies are special because they have the ability to activate certain immune cells called mast cells. Mast cells are like the body’s alarm system. When activated by IgE antibodies, they release chemicals that cause the symptoms of an allergic reaction, such as itching, swelling, and difficulty breathing.
The next time the person eats peanuts, even a tiny amount, their immune system quickly recognizes the peanut proteins and activates the mast cells, leading to an allergic reaction. This is why people with peanut allergies must be very careful to avoid exposure to peanuts.
Sometimes, the first exposure to peanuts that leads to sensitisation can be very small and go unnoticed. This is why some people may not realize they have a peanut allergy until they have a more obvious reaction.
Adrenaline autoinjector use
Peanut allergies can be very serious because even a small amount of peanut can cause a severe allergic reaction in some people.
This is why carrying an adrenaline autoinjector (e.g. an EpiPen or AnaPen) at all times is very important for people with these types of allergies, as it is not always possible to be sure that all food is allergen free.
Most IgE mediated allergic reactions will occur very quickly – within one hour following exposure but occasionally it can take up to four hours.
Symptoms
The symptoms of an IgE mediated allergic reaction can be swelling of the face, lips or eyes, hives, redness of the skin or abdominal pain, vomiting and trouble breathing.
Anaphylaxis
Anaphylaxis is the most severe form of IgE mediated allergy. Symptoms generally occur rapidly after exposure to an allergen, but in some cases there can be a delay. Anaphylaxis is life threatening and an adrenaline autoinjector (e.g. EpiPen) should be used if it develops.
Symptoms
The symptoms of anaphylaxis include: tongue swelling, throat scratchiness and swelling, difficulty talking, noisy breathing, wheeze, difficulty with breathing, persistent dizziness and collapse, paleness and floppiness in children.
Non-IgE mediated reactions
There are a number of common chemicals that the body does not tolerate, but these do not involve the IgE mediated immune response. The symptoms generally develop hours to days after ingestion and are mainly gastrointestinal such as bloating and diarrhoea. Because IgE is not involved, anaphylaxis does not develop. An adrenaline autoinjector (e.g. EpiPen) is not required. These reactions generally occur to foods and can be diagnosed on history and then elimination from the diet, rather than from blood or skin prick tests.
Diagnosis of allergies
In some cases, the diagnosis of a food allergy can be straight forward.
If there has been a typical allergic reaction (lip swelling, itchy eyes, throat symptoms, rash) immediately following the ingestion of a type of food that is a common allergen such as peanut, prawns or other shellfish, you can be reasonably sure that the food is the cause of the allergic reaction.
However sometimes this type of reaction can occur when a person has had a meal at a restaurant and is not certain of all the ingredients present, or has had the food many times before without having a reaction. It can then take some detective work to try to pinpoint whether it was indeed a food allergen or whether it may have been idiopathic (occurs due to an unknown trigger) or whether there is another cause of the symptoms.
Environmental allergens
Environmental allergens such as dust mite, pollen and animal dander, may produce very different symptoms. People may experience a blocked or runny nose (allergic rhinitis) and itchy eyes (allergic conjunctivitis) and may not be able to pinpoint exact triggers. Sometimes people with these symptoms will have been referred by other specialists such as ENT surgeons.
Allergies can also complicate the management of other conditions such as asthma and eczema, and correct identification and management of allergie can help stabilise these conditions.
Sometimes people have experienced allergic reactions to medications such as antibiotics or anaesthetic agents, and this needs to be confirmed before these drugs can be safely used again. Skin testing or a blood test may initially be performed, but in some cases an oral challenge is necessary to be completely sure that an allergy is not present.
How allergy testing works
After a detailed history and physical examination, it will be clear whether allergy testing is required or not. Skin prick testing is the most common method for testing of IgE mediated allergies (i.e. not intolerances) such as peanut allergy, dust mite allergy, grass allergy etc.
It is important not to have taken antihistamines in the 72 hours prior to the skin prick testing as this can affect the result.
The testing will occur in the consultation room. The skin of the forearm will generally be used. The skin is pricked with a disposable lancet for a positive (histamine) control, negative control, and the allergens being tested. This is based on the history and the likelihood of cross-reactivity.
Cross-reactivity refers to the likelihood that a person who is allergic to one protein will also be allergic to related proteins.
It may take up to 10 minutes for a welt to appear around the skin prick, and then the response to the allergen will be measured and recorded. The results will then be explained and a discussion will occur on what treatment options are available.
Contact dermatitis and patch testing
Contact dermatitis is a type of skin inflammation caused by direct contact with a substance that irritates the skin or triggers an allergic reaction. Common culprits include certain metals (like nickel), cosmetics, fragrances, and some plants. If you suspect you have contact dermatitis, your doctor may recommend patch testing. During this procedure, small amounts of potential allergens are applied to patches and placed on your back. The patches remain in place for 48 hours, and you’ll need to return to the office for patch removal and an initial reading. Another visit 48 hours later is necessary to check for delayed reactions. Patch testing helps identify the specific substances causing your contact dermatitis, allowing you to avoid those allergens and find relief from symptoms like redness, itching, and rash.
When not to test…
Allergy testing should be the next step in investigation of a person’s symptoms after a thorough history and examination has taken place, as a detailed history in many cases can narrow down the likely allergens that need to be tested.
This is important so that the tests can be interpreted correctly. Allergy testing should not be done as a screening test. If a large number of allergens are tested without a clear reason, there may be some that are mildly positive – but if a person has been eating that food without any allergic reaction, it is likely that this is a “false positive” rather than a true allergy. This can lead to the need for further and more costly testing, or unnecessary avoidance of a food.
A negative skin prick test is also not a guarantee that there is no problem with a food – it just may be a food intolerance syndrome rather than an IgE mediated allergy. Some of these syndromes are very classic and can be diagnosed on history alone, avoiding the cost and discomfort of testing.