An allergy is a hypersensitive response to a substance that is normally harmless, but which produces an immune reaction in the sufferer’s body.
An allergy may be immediate-type (for example allergy to cats or peanuts) or delayed-type (for example allergy to hair dye or gel nails). Immediate-type allergy is investigated by prick testing and delayed-type allergy by patch testing.
Immediate-type allergic reactions can range from a runny nose to anaphylaxis.
In general, ingested (food) allergies will produce immediate skin or gastrointestinal symptoms, or anaphylaxis, and inhaled allergens (such as pollens) will produce immediate respiratory or ocular symptoms. Both ingested and inhaled allergens can, however, produce the full spectrum of allergy symptoms.
Prick testing for immediate-type allergy
Immediate-type allergic reactions are usually investigated by prick testing, generally carried out on the arms, with results available after 10-20 minutes. Prick testing is painless. If allergy to foods is suspected, it is useful to test the relevant fresh foods like individual nuts and raw fruits, which patients are requested to bring with them. Many food solutions are also available at the hospital for testing.
Occasionally blood tests may be necessary, to look for specific serum immunoglobulin (Ig)E antibodies. Until recently the blood test used was a RAST (radioallergosorbent) test, but more recently the IgE-specific immunoassay technique has taken its place. Blood tests can sometimes be useful if a careful history and prick testing have not been decisive in diagnosing or ruling out an allergy. The results of blood tests are delayed and are often difficult to interpret meaningfully. Total serum IgE is often raised in individuals with allergies and makes the results of specific IgE difficult to interpret. Component-resolved diagnostic tests, however, can provide some predictive value, for example, in patients with nut allergy.
Some relevant food allergies triggering severe lifelong eczema are non-IgE mediated, hence both prick tests and blood tests may not be positive.
There are no tests for food intolerance(s).
Some allergies can be managed through a change in lifestyle and medication. The aim is to bring allergy symptoms under control and to improve quality of life.
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