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Laboratory Diagnosis of “True” Peanut Allergy

Peanut is as common as egg allergy and represents an extremely potent allergen that
can result in severe life threatening reactions. The awareness of peanut allergy and its
potential danger has increased the demand for peanut testing. One of the most
common tests used to help identify clinical peanut allergy is peanut specific IgE. This
test has been shown to have utility as a tool to prevent severe reactions in children
when given an oral peanut challenge. However, the use of peanut specific IgE does
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not perform well as a diagnostic tool for clinical peanut allergy . Studies to differentiate
peanut sensitized patients as defined by the presence of peanut specific IgE and
clinically allergic peanut patients have shown that only about 20% of peanut sensitized
patients actually have peanut allergy. Investigations in many countries have
demonstrated that the blood test for the purified peanut allergen, Ara h 2, is highly
specific and sensitive for clinical peanut allergy. In order to help identify “true” peanut
allergy, a panel of specific IgE tests has been developed by Allermetrix for physicians
as an aid to evaluate potential risk for clinical reactions.

Peanut Allergens

Peanuts when extracted contain many different proteins. At the time of writing there
have been 13 allergens described. A peanut allergen is a protein that has been
demonstrated to bind to IgE from a peanut allergic individual. Each allergen is named
using the genus and species of the source material, Arachis hyopgaea for peanut, and
by its order of discovery. Therefore the peanut allergens are named Ara h 1 through
Ara h 13, where Ara h 1 was discovered first and Ara h 13 is the most recently
described. Ara h 4 was found after discovery to be the same as Ara h 3, and the Ara h
4 name is no longer used.

Some of the individual allergens like Ara h 1 also have variations due to post-
translational modifications like glycosylation. Some of the proteins are glycosylated
differently from each other and some not at all. From these differences in post-
translational modifications arise isoforms. Some individual protein allergens may have
several isoforms, and each may react differently to IgE.

Some of these peanut allergens are very similar to allergens in different source
materials (e.g. other foods and plant pollens). Cross-reactivity occurs when IgE that
binds a peanut allergen also binds a similar allergen in other sources. It is a
characteristic of IgE antibodies that are formed in response to allergens that share
similar structures with allergens from other sources. Cross-reactivity is important to
recognize because the IgE that binds an allergen could have been formed to a cross-
reactive source and not to the allergen being tested. In some instances this cross-
reactivity will result in clinical symptoms and not in others. Identifying cross-reactions
can add to the clinical picture for a patient by ascertaining whether the presence or
absence of symptoms can be linked to the cross-reactive source.

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