Allergy refers to immune-system hypersensitivity to allergens.
Allergens are proteins that the immune system recognises as foreign and potentially harmful, leading to an allergic immune response.
Antigens are proteins that can be recognised by the immune system. The body will come in contact with millions of different antigens, and very few will lead to a hypersensitivity reaction.
Atopy describes a genetic tendency to develop IgE-mediated allergic diseases, such as eczema, hayfever, allergic rhinitis, food allergies and allergic asthma. These conditions are referred to as atopic conditions.
Hypersensitivity Reactions
The Coombs and Gell classification is used to categorise hypersensitivity reactions.
Type 1 hypersensitivity reactions involve IgE antibodies to a specific allergen, triggering mast cells and basophils to release histamines and other cytokines. This causes an immediate reaction. Examples are food allergies, insect sting allergy, anaphylaxis and allergic rhinitis.
Type 2 hypersensitivity reactions involve IgG and IgM antibodies binding to antigens, activating the complement system, and causing cell damage. Examples are haemolytic disease of the newborn and transfusion reactions.
Type 3 hypersensitivity reactions involve the accumulation of immune complexes that damage local tissues. Examples are systemic lupus erythematosus (SLE) and post-streptococcal glomerulonephritis.
Type 4 hypersensitivity reactions involve cell-mediated reactions caused by T lymphocytes. T-cells are inappropriately activated, causing inflammation and damage to local tissues. Examples are organ transplant rejection and contact dermatitis.
Presentation
IgE-mediated food allergy symptoms typically develop within minutes to 2 hours after ingestion. They vary in severity between patients. Affected systems may include:
- Skin symptoms (e.g., itching, urticaria and angioedema)
- Respiratory symptoms (e.g., cough, wheeze and breathlessness)
- Gastrointestinal symptoms (e.g., nausea, vomiting and diarrhoea)
- Systemic symptoms (e.g., tachycardia, hypotension and confusion)
History
A detailed history is the most important aspect of diagnosing allergies. Allergies can be diagnosed on history alone, and investigations are difficult to interpret accurately without a clear history. Important areas to cover include:
- Timing after exposure to the allergen
- Previous and subsequent exposure and reaction to the allergen
- Symptoms of rash, swelling, breathing difficulty, wheeze and cough
- Previous personal and family history of atopic conditions and allergies
Investigations
There are three key ways to test for allergy:
- Skin prick testing
- Serum allergen-specific IgE testing
- Food challenge testing
Skin prick testing involves making a small break in the skin with a sample of a suspected allergen. A patch of skin is selected, usually on the forearm. Strategic allergen solutions are selected (e.g., peanuts, house dust mite and pollen). A drop of each allergen solution is placed at the marked points on the patch of skin, along with a saline control and a histamine control. A single-use lancet is used to make a tiny break in the skin at the site of each allergen. After 15 minutes, the size of the wheals to each allergen is assessed and compared to the controls.
Serum allergen-specific IgE testing measures the amount of IgE to specific allergens (e.g., peanuts) in a blood sample. In patients with atopic conditions, such as eczema and asthma, the results are often positive when they do not have a clinical allergy.
Food challenge testing should be performed in a specialised unit with very close monitoring. The child is given gradually increasing quantities of the suspected food allergen and closely monitored after each exposure. This can be helpful for excluding allergies and providing reassurance.
Skin prick testing and serum IgE testing assess sensitisation and do not necessarily indicate clinical allergy. This can make the results unreliable and misleading. They often come back showing that the patient is sensitised to many allergens, and it can be difficult to determine which results are clinically significant. It can be challenging to explain to the child and their parents that the positive test results do not mean it is unsafe for the child to eat those foods.
Food challenge testing is the gold standard investigation for food allergy. However, it requires a lot of time and resources and is only available in selected places.
Patch Testing
Patch testing is used to test whether a specific allergen is causing allergic contact dermatitis. The patient could be tested for reactions to metals, perfumes, or chemicals. A patch containing the allergen is placed on the patient’s skin. The patch can either contain a specific allergen or a grid of many allergens as a screening tool. After 2-3 days, the skin reaction to the patch is assessed.
Management
Management of food allergy involves:
- Carefully establishing the correct allergen
- Avoiding the allergen
- A written allergy management plan
- Adrenaline auto-injector (e.g., EpiPen), depending on the risk of anaphylaxis
Specialist referral should be considered to:
- Confirm the diagnosis
- Perform allergy testing
- Manage complex cases
Treatment of accidental exposure with mild to moderate symptoms involves:
- Non-sedating antihistamines (e.g., cetirizine or loratadine)
Management of severe, systemic reactions (e.g., anaphylaxis) involves:
- Intramuscular adrenaline
- Emergency admission
In certain scenarios, immunotherapy (or desensitisation) is an option. Specialist centres may initiate a lengthy process of gradually exposing the patient to increasing doses of an allergen over months to reduce their reaction.
Last updated April 2026
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