Specific IgE

Tests that detect antigens and antibodies use similar immunological principles, relying on the antibody-antigen reaction. They are referred to as immunoassays or immunochemical techniques. All specific IgE molecules belong to one class or isotype of antibody. The other isotypes are IgG, IgA and IgM antibodies. Even though they have variable antigen-binding regions, giving them their specificity, they have an effector area that is constant. Antibodies can be raised, using monoclonal techniques against this constant area, producing anti-IgE antibodies.

The allergen is presented to the serum in a variety of forms. The antibodies within the serum then either react or do not react with the presented allergen. The reacting antibodies are then 'labelled' by the binding of a detection, anti-IgE, antibody to the serum IgE.

Both the appropriateness and standardisation of the allergen are important in this diagnostic test. Clinical validation depends upon sera from individuals with DBPCFC-proven food allergies. This can be problematic when developing assays for the investigation of rare food allergies.

Different methods use different allergen presentation systems (fluid phases, microparticles and various solid phase presentation vehicles such as polystyrene or paper), and different anti-IgE detection 'labelling' systems (radioactivity, enzyme reactions, and enzyme systems linked with fluorescence and chemiluminescence).29 The radioallergosorbent test (RAST) and the CAP radioimmunoassay (RIA) systems exploit radioactivity as the labelling mechanism, with various antigen presentation systems within different kits. The CAP FEIA system uses the principle of enzymes linked with fluorescence as its labelling system, having presented the antigen on a cellulose 'sponge'.

An international scale for the in vitro quantification of IgE has proved difficult to produce. This is because different sera from different allergic patients demonstrate different quantitative responses, in particular as different patients react to different epitopes on each allergen.

Two systems are used in practice for the quantification of specific IgE, though both are prone to error. They do show that a reasonable degree of correlation is obtained between different detection systems. Reports vary concerning the concordance of results from different laboratories and different systems. They range from up to 90% of laboratories with concordant results for common inhalant and food allergens to reports from other authorities of much lower concordance when comparing the various detection systems. Bindslev-Jensen and Poulsen quote figures for different methods of detecting specific IgE to different food allergens. They list high levels of specificity, between 80% and 90% and even approaching 100%, for cod. When they examined specific IgE to cereals, sensitivity was not quoted and specificity was very low, the lowest being 16% for one of the systems for rye. The sensitivity with different systems for hazelnut was only about 50%. As researchers have not been able to discover a 'level of discrimination' above which patients will have clinical symptoms and below which such symptoms are unlikely to occur, manufacturers have opted for high sensitivity at the expense of specificity. It is clear from these comparisons that a particular system cannot be recommended for all food allergens, as the results of each test need to be taken in the context of both the particular allergen and the system used to identify the specific IgE.30

It is clear that some allergens share common epitopes (areas of the protein recognised by each specific IgE molecule). This has implications for both clinical cross-reactivity and cross-reactivity in vitro. Specific IgE may bind with a related allergen with a common epitope. If a patient has clinical problems with a particular allergen, what is the likelihood that they will have problems with other allergens sharing these common epitopes? Epitopes which are heat and acid resistant are more likely to have clinical relevance. The major allergen present in codfish, Gad c1, demonstrates remarkable heat and acid resistance. Individuals should be advised to avoid cross-reacting species of fish. The cross-reactivity in vitro that is evident between grass pollen and other food cereals such as wheat flour does not seem to be clinically relevant.30

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