This site is intended for health professionals only

Allergy and allergy testing: an introduction

Sue Cross
RN BSc(Hons) NPDip PGCE
International Project Manager
Respiratory Education Resource Centres
University Hospital Aintree
Liverpool
E:sue@respiratory erc.com

Confusion often arises between the terms "atopy" and "allergy". Atopy is defined as the production of specific immunoglobulins (IgEs) in response to exposure to common environmental allergens, such as house dust mites, grass and cat dander.(2) This will then be shown if the patient has a positive reaction to an allergy test. Being atopic, however, doesn't necessarily mean that the patient is allergic.
It is only when the patient demonstrates clinical symptoms of an allergic disease, as well as having a positive allergy test, that allergy avoidance advice and treatment should be given.
 
Diagnosing allergy
Allergy to environmental agents can affect almost every organ of the body. Although allergic rhinitis is the commonest manifestation, the lower respiratory tract, the conjunctiva, the skin and the gastrointestinal tract are frequently affected by allergic disease.
As allergic diseases are common and their prevalence is increasing, accurate diagnosis of triggering or causative allergens is essential for appropriate advice on avoidance and environmental control measures. Although allergic diseases can occur at almost any age, some allergies are most likely to develop for the first time in particular age groups

Symptoms
In many cases an immediate relationship exists between exposure to potential allergens and the development of symptoms, making both the diagnosis and identification of allergy straightforward. However, in 25-50% of cases the predominant symptoms develop one to 10 hours after exposure (late phase reactions), obscuring the allergic nature of the illness and making diagnosis more ­difficult. In allergic diseases of occupational ­aetiology, the first symptom may be exercise-induced asthma, or night-time waking with cough. The longer such symptoms have been present, the more likely they are to persist when ­exposure ceases.

Allergy tests

Serum allergen-specific IgE concentrations
In the ImmunoCAP test, allergens are chemically bound to a solid phase, which is a sponge-like cellulose polymer encased in a capsule. The sponge is used because of the high surface area, which can bind a high volume of protein. This is particularly useful because allergens are very complex and have many forms or epitopes. The sponge matrix allows presentation of all of these in high concentrations and is therefore a sensitive and accurate assay system. Over 450 different allergen tests are available.
When the patient's serum is added, allergen-specific IgE binds to the immobilised allergen. Anti-IgE labelled with fluorescent label is then added, which attaches to specific IgE in the sample. The amount of specific IgE to each allergen can accurately be determined from the amount of fluorescence that is generated and is expressed as a quantitative result.

Skin prick tests
The skin prick test can be performed during the initial consultation with aqueous solutions of a variety of allergens. The test should be performed with standardised allergen solutions if possible. In general practice it may be sufficient to use four allergens (house dust mite, grass pollen, cat and dog allergen) plus the positive and negative controls to confirm or exclude atopy and recognise the most common allergens encountered.(3)
In the next part of this series, the importance of taking a case history and physical ­examination in ­diagnosing for allergy will be discussed.

Referrences
1. Kay B. Good allergy practice. In: Durham S, editor. ABC of allergies. London: BMJ Publishing Group; 1998.
2. Coca AF, Cooke RA. On the ­classification of the phenomenon of hypersensitivity. J Immunol 1923;8:163.
3. Frew AJ. Skin tests. In: Kay B, editor. Allergy and allergic diseases. Oxford: Blackwell Science; 1997.

Resources
British Allergy Foundation
W:www.allergyfoundation.com
Allergy Diagnostic Centre
W:www.pathlynx.co.uk/adc