We asked Consultant Paediatrician, Dr Chinedu Nwokoro: What is Allergy?
Allergy is present when the immune system produces a harmful or irritant reaction to food, airborne or other trigger substances (known as allergens).
Is it the same as Food Intolerance?
No. Food Allergy is different from food intolerance. Food intolerance may be caused by other non-allergic causes. These include:
Lactose intolerance – where low levels of the enzyme that breaks down milk protein in the gut wall (lactose deficiency) can cause bloating, diarrhoea, constipation and pain.
Coeliac disease – where the presence of gluten (found in wheat flour) in the diet causes the body to mount an immune reaction against itself (an autoimmune response) and damages the lining of the gut, causing similar symptoms.
Irritable bowel syndrome – this is a poorly understood condition of altered bowel habit and sensation with no physical evidence of gut damage.
What are the main types of Allergy?
Allergic reactions (also known as hypersensitivity) can loosely be divided into immediate (Type 1) allergy or delayed type allergy.
These occur within 2 hours of exposure to an allergen, and are caused by overproduction of Immunoglobulin E (IgE) by the immune system in response to the allergen. They tend to be more severe, and can include:
– red, swollen itchy rash (doctors call this urticaria)
– swelling of the face and lips (doctors call this angiooedema)
– swelling of the intestines, causing abdominal pain, vomiting, pallor and diarrhoea, and occasionally collapse
– inflammation of the upper airway, causing sneezing (rhinitis), nose blockage or discharge (rhinorrhoea), itchy eyes (conjunctivitis), cough, noisy breathing and throat tightness
– inflammation of the lower airway, causing wheeze, difficulty in breathing, chest tightness and rarely collapse
Delayed type reactions
These occur within 48 hours of exposure to allergen, are not caused by IgE, and tend to be milder. They include:
– development or worsening of eczema
– gastro-oesophageal reflux
– loose, frequent or bloody stools (may cause nappy rash)
– tummy pain (may present as colic, food refusal)
– being pale and tired
Mixed type reactions
Some children will exhibit both types of reaction to the same allergen
Where these reaction are severe enough to affect breathing or conscious level they are called anaphylaxis and are treated with intramuscular adrenelin.
– Anaphylaxis risk cannot be predicted by skin prick test wheal size
– Anaphylaxis can occur despite previous mild reactions to the same allergen
– Delayed type reactions do not lead to anaphylaxis
How can Allergy be identified
1. The Allergy-focused history
– Your doctor will begin by asking detailed questions about the type, timing, and nature of any reactions and other medical conditions such as asthma, hay fever and eczema
– This provides a sense of the likelihood of allergy, and the safety of doing testing
– It also guides the range of tests to be performed
2. Allergy Skin Prick Testing
– This test identifies IgE-mediated (Immediate type) allergy and allergic sensitisation
– It is not valid without a supporting allergy history (see above)
– It is possible to be IgE-sensitised to a food without being allergic to it
– Children should be well, in particular not suffering from wheeze at the time of the test
– They should have been off all antihistamine for 72-96 hours
– Skin should be clear of steroid or moisturiser (emollient) creams
– Results are available the same day
How is it done?
Equipment to promptly treat anaphylaxis (injectable adrenalin and inhaled salbutamol) or minor reactions (1% hydrocortisone and cetirizine liquid or tablet) MUST be available before beginning
– A labelled grid is drawn on the arm
– Allergen extracts and positive (histamine) and negative (salt solution) controls are placed 2cm apart on the arm
– Each extract is pricked lightly with an individual lancet (no blood is drawn)
– The skin is blotted dry
– The diameter of the wheal (the raised area of skin – excluding any little projections or pseudopodia) is measured 15 minutes later and recorded on a pro forma.
The test is invalid if:
– the negative control shows a reaction (this indicates dermatographia or reactive skin) – in this case blood testing should be performed
– the positive control does not show a reaction (this indicates prior use of antihistamine, topical steroid cream or rarely an immunodeficiency) – again, blood testing may be helpful
3. Specific IgE Blood Testing
– This test tests the amount of Immunoglobulin E (IgE) in the blood that reacts to a particular allergen.
– It can be used regardless of antihistamine use
– High levels of background total IgE may cause false positive tests
– Blood can be sent for boat range microarray specific IgE testing (the ISAC test) where doubt about the likely allergen coexists with a strong suspicion of significant allergy
– Results can take several weeks to come back
– Neither this nor skin prick testing will identify non-IgE-mediated allergy
4. Food Elimination Diet
– Delayed type allergy can be identified by specific exclusion of suspected allergen from the diet for 6 weeks and subsequent reintroduction if appropriate
– The support of a dietitian may be of value in this scenario
– There are no laboratory tests for delayed type food allergy
5. Food Allergen Challenge
– When it is felt that a child is likely to have outgrown his food allergy then allergy challenge may be appropriate
– This is usually performed with the back-up an inpatient medical ward and would not routinely be performed at this hospital
How can allergy be treated?
There is good evidence that early exposure to potential food allergens is protective against the development of allergy in susceptible children.
Therefore early exposure to as wide a range of foods as is practicable in early life (including in the womb and via breastmilk) is recommended in the majority of well children.
This article by the American Academy of Asthma, Allergy and Immunology (AAAAI) may be of use.
There is no evidence of benefit from avoiding allergenic foods in pregnancy, while breastfeeding or in early infancy in children who are not already allergic.
Children with established food allergy should avoid their allergic triggers, taking care to maintain adequate nutrition with the assistance of specially trained paediatric allergist
Removal of aeroallergens (animals in particular) can improve other related conditions such as asthma. The evidence for benefit from attempts to reduce house dust mite exposure is poor.
Treatment of acute symptoms
Mild symptoms can be treated with oral antihistamine such as cetirizine of chlorphenamine
Children with history or high risk of anaphylaxis should be provided with an adrenalin autoinjector for school and home, and an anaphylaxis plan to be shared with their school/childcare setting.
Treatment of longer term complications
Control of asthma cannot be achieved without management of aeroallergen (e.g. treatment of hay fever, pet allergy) and food allergen triggers.
Similarly uncontrolled asthma dramatically increases the risk of anaphylaxis in response to allergen exposure
Desensitisation therapy is available and may rarely be of benefit in reducing allergic sensitisation. We can refer suitable patients if needed but do not currently provide it at this hospital.
If you are concerned about your child Dr Nwokoro is available at One Hatfield Hospital either by booking directly through our Reservations Team on 01707 44 33 33 or by referral from your GP
One Hatfield is unique providing diagnostic care and same or next day appointments, and is ideally located for patients in Hatfield, St Albans, Stevenage, Watford, North London and throughout Hertfordshire and Bedfordshire.