To effectively manage someone’s allergies, it is important to know what he/she is allergic to so that it can be avoided. A simple method of doing this is to expose the individual to a range of potential antigens by placing a drop of allergen extract on the patient’s skin and pricking through the drop. If the person is allergic (atopic) to any of the applied substances then the area will become red and slightly raised – a wheal and flare reaction.
An allergic reaction is initiated when IgE antibodies encounter ‘harmless’ environmental molecules called allergens for which they have a single specific affinity. These IgE antibodies are concentrated on mast cells. When they are cross-bridged by the allergen the mast cell releases chemicals that generate the allergic symptoms. The laboratory test to detect what specific substances cause allergies doesn’t measure the outcome of a controlled allergic response as exposure tests do but instead measures the amount of specific IgE with affinities for known allergens. Patient serum that has been separated from clotted red blood cells is added to a cellulose polymer sponge that has been impregnated with either a single or mix of allergens. Any IgE present, that is specific for the allergens, will bind to them whilst the rest is washed off. Next another enzyme-tagged antibody is added that binds to all IgE. A substrate is then added that reacts with the enzyme producing a colour change that can be quantified and compared with the colour change from reactions with known concentrations of specific IgE to calculate a result for the patient. The calculated quantities of specific IgE are then compared with the amount of specific IgE that people without allergies to the substance in question expect to have.
This test is generally automated with the majority of laboratories using an assay called IMMUNOCAP, which produces accurate standardised results and can identify specific IgE to many different allergens.
Allergy consultants may often use an exposure test to diagnose a patient with mild allergic symptoms. In some cases though, the risk of a causing a severe allergic response by performing this kind of test is considered too great, so an indirect laboratory approach is used instead. This may be the case if a patient has presented at and Accident and Emergency department with severe allergic symptoms. An acute allergic response affecting many parts of the body at once is called anaphylaxis and can lead to many symptoms including rashes, swelling of the airways, vomiting, itching, reduction in blood pressure and it can even result in death. The initial response here is to reduce the symptoms of anaphylaxis by giving the patient adrenaline and anti-histamines. After the patient’s condition has stabilised the doctors can begin to investigate exactly what caused the severe reaction. In this incident, the clinician in charge may suspect that something in the meal the patient recently ate lead to the anaphylaxis but because the reaction was so severe it would not be wise to expose the patient again in a skin prick test. The immunology laboratory would therefore be asked to perform a specific IgE quantification using the IMMUNOCAP technology. The patient’s serum would be tested first against a mixture of common food allergens including peanut, if this was positive, the patient could then be tested against each of the individual allergens. If the final result showed that the patient had elevated levels of IgE specific for peanut then the clinician would advise avoiding further exposure and also show them how to inject themselves with adrenaline using a device called an Epipen in case of accidental exposure.