Allergy is an umbrella term for hypersensitivity of the immune system to allergens. Allergens are proteins that the immune system recognises as foreign and potential harmful, leading to an allergic immune response. These proteins are types of antigen. 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. The ones that do are called allergens.

Atopy is a term used to describe a predisposition to having hypersensitivity reactions to allergens. It refers to the tendency to develop conditions such as eczema, asthma, hayfever, allergic rhinitis and food allergies.  These conditions are referred to as atopic conditions. Patients often have more than one atopic condition, and atopy frequently runs in families.


The Importance of Establishing and Excluding Allergies

Having a food allergy can be a huge psycho-social burden, particularly in those who have anaphylaxis or an epipen. It means checking all food labels for ingredients, ensuring all those responsible for the child are aware (e.g. school, other parents and relatives) and being very cautious or avoiding eating out in restaurants or anywhere with unlabelled food, where allergens may have made their way into foods.

It is not uncommon for symptoms and histories of “allergy” to actually be a somatisation disorder rather than a true allergy. It is important to establish whether symptoms are down to an allergy, or more psychological, because an allergy diagnosis can lead to restrictive or unhealthy eating and do more harm than good. Allergy testing can play a role in reassuring patients that they do not have a true allergy to certain foods.


The Skin Sensitisation Theory of Allergy

The skin sensitisation theory is currently the leading theory on the origin of allergies. This theory suggests there are two main contributors to a child developing an allergy to a food:

  1. There is a break in the infant’s skin (from eczema or a skin infection) that allows allergens, such as peanut proteins, from the environment to cross the skin and react with the immune system.
  2. The child does not have contact with that allergen from the gastrointestinal tract, and there is an absence of GI exposure to the allergen.

The theory is that allergens entering through the skin are recognised by the immune system as being foreign and harmful proteins. The immune system reacts by becoming sensitised to that allergen, so that when it next encounters that allergen again it will launch a full immune response (an allergic reaction).

When a baby is weaned at around 6 months, if they are regularly eating foods that contain that allergen, their GI tract is regularly being exposed to that protein. The GI tract will recognise that allergen as a food and not a foreign or harmful protein, and inform the immune system that it is a safe thing to be exposed to.

The theory is that regular exposure to an allergen through food and preventing exposure to that allergen through the skin barrier can help prevent food allergies developing.


Hypersensitivity Reactions

Many conditions are a result of hypersensitivity reactions:

  • Asthma
  • Atopic eczema
  • Allergic rhinitis
  • Hayfever
  • Food allergies
  • Animal allergies


The Coombs and Gell classification is used to describe the underlying pathology of different hypersensitivity reactions:

Type 1: IgE antibodies to a specific allergen trigger mast cells and basophils to release histamines and other cytokines. This causes an immediate reaction. Typical food allergy reactions, where exposure to the allergen leads to an acute reaction, range from itching, facial swelling and urticaria to anaphylaxis.

Type 2: IgG and IgM antibodies react to an allergen and activate the complement system, leading to direct damage to the local cells. Examples are haemolytic disease of the newborn and transfusion reactions.

Type 3: Immune complexes accumulate and cause damage to local tissues. Examples are autoimmune conditions such as systemic lupus erythematosus (SLE), rheumatoid arthritis and Henoch-Schönlein purpura (HSP)

Type 4: Cell mediated hypersensitivity 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.



A detailed history is the most important aspect of diagnosing allergies. Allergies can be diagnosed on history alone and investigations are impossible to accurately interpret 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



The most reliable information about whether a patient has an allergy is a clear and detailed history.

There are three main ways to test for allergy:

  • Skin prick testing
  • RAST testing, which involves blood tests for total and specific immunoglobulin E (IgE)
  • Food challenge testing

Skin prick testing and RAST testing assess sensitisation and not allergy. This is important, because it makes these tests notoriously unreliable and misleading.

Think carefully before performing an allergy test, particularly a RAST test. They often come back showing that the patient is sensitised to many of the things you have tested for, and it becomes very challenging to explain to the child or their parents that the positive test results do not mean it is unsafe for the child to eat those foods.

Foot challenge testing is the gold standard investigation for diagnosing allergy, however it requires a lot of time and resources and is only available in selected places. 


Skin Prick Testing

A patch of skin is selected, usually on the patients forearm. Strategic allergen solutions are selected, for example peanuts, house dust mite and pollen. A drop of each allergen solution is placed at marked points along the patch of skin, along with a water control and a histamine control. A fresh needle 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 are assessed and compared to the controls.


Patch Testing

Patch testing is the most helpful in determining an allergic contact dermatitis in response to a specific allergen. It is not helpful for food allergies. This could be for latex, perfumes, cosmetics or plants. A patch containing the allergen is placed on the patient’s skin. The patch can either contain a specific allergen, or a grid of lots of allergens as a screening tool. After 2 – 3 days the skin reaction to the patch is assessed.


RAST testing

RAST testing measures the total and allergen specific IgE quantities in the patient’s blood sample. In a patient with atopic conditions such as eczema and asthma, the results will often come back positive for everything you test.


Food Challenge

A food challenge should be performed in a specialised unit with very close monitoring. The child is gradually given increasing quantities of an allergen to assess the reaction, starting with almost non-existent quantities diluted further in other foods, for example mixing a small amount of peanut into a bar of chocolate. Children are monitored very closely after each exposure. This can be very helpful in excluding allergies for reassurance.



  • Establishing the correct allergen is essential
  • Avoidance of that allergen
  • Avoiding foods that trigger reactions
  • Regular hoovering and changing sheets and pillows in patients that are allergic to house dust mites
  • Staying in doors when the pollen count is high
  • Prophylactic antihistamines are useful when contact is inevitable, for example hayfever and allergic rhinitis
  • Patients at risk of anaphylactic reactions should be given an adrenalin auto-injector

In certain cases, specialist centres may initiate a lengthy process of gradually exposing the patients to allergens over months, called immunotherapy, with the aim of reducing their reaction to certain foods or allergens.


Following Exposure

Treatment of allergic reactions are with:

  • Antihistamines (e.g. cetirizine)
  • Steroids (e.g. oral prednisolone, topical hydrocortisone or IV hydrocortisone)
  • Intramuscular adrenalin in anaphylaxis

Antihistamines and steroids work by dampening the immune response to allergens. Close monitoring is essential after an allergic reaction to ensure it does not progress to anaphylaxis.


Last updated January 2020