Skin tests to insects: reliable markers of sensitization?

Positive skin tests to natural allergen extracts are thehallmark of immediate hypersensitivity and play a keyrole in allergy diagnosis. Even today, most allergydiagnosis is based on skin tests, rather than in vitromeasurements of allergen-speciﬁc immunoglobulin(Ig)E. Skin tests have been extensively used to assesssensitization in epidemiological studies, especially inchildren, where they allow sensitization to be measuredquickly and relatively painlessly, without the need todraw blood. Skin tests are sensitive and show greaterspeciﬁcity with quality allergen extracts (mites, pollens,animal danders). Cross-reactivity between IgE anti-bodies to allergens can complicate skin test results(reviewed in 1), especially in epidemiological studieswhere there may be little clinical follow-up. Theseproblems may be exacerbated with less well-deﬁnedextracts such as those derived from insects, fungi orfoods, which are unstandardized and may containallergens that cross-react with mites or pollens.A short communication by Lodrup Carlsen andcolleagues in this issue of the Journal describes a highprevalence of cockroach (CR) sensitization amongasthmatic children and their parents in Norway (2). Of100families(426subjects)enrolledintheGeneticsofAsthmaInternationalNetwork(GAIN)study,andwhowere skin tested with common inhaled allergens, 7.5%(31 subjects) were sensitized to CR as assessed by skin-prick testing (SPT) (>3 mm wheal) or serum IgEantibodies. Among sensitized subjects with at least onepositive skin test, 14% were CR sensitive. However,only three of the CR-sensitized subjects gave a >3mmskin test and had detectable serum IgE ab (> class 2). These ﬁndings are unexpected because there hasneither been previous reports of CR allergy inScandinavia, nor of CR allergens in housing. At issueiswhethertheseobservationsreﬂecttruesensitizationtoCR allergens, or reﬂect IgE cross-reactivity with otherenvironmental allergens to which the subjects may havebeen exposed. The lack of a correlation between skintests and serum IgE ab responses suggests either thatthe patients were weakly sensitive to CR or that theskin-test results might reﬂect cross-reactivity to otherallergens. The results do not seem to ﬁt the pattern ofresponses seen in CR allergic patients in the US, forexample, who show positive skin tests to CR, have highlevels of serum IgE ab (CAP class 3–6) and usually livein CR-infested houses or apartments (3–6). Similar CRallergic asthma patients, living in low cost publichousing in Strasbourg, France, gave positive skintests to CR extracts and were exposed to high levelsof Bla g 1 and Bla g 2, both in dust and airborne in theirapartments (7). Other cross-sectional surveys in Europehave reported an apparently high prevalence (12–25%)of positive skin tests to CR (reviewed in 6), but theexposure to CR was not studied.Lodrup Carlsen and colleagues investigated whethercross-reactivity could be a factor in their study butfound only three cases where the subjects also hadserum IgE ab to shrimps, which might be explained bycross-reactive antibodies to tropomyosin. The tropo-myosin allergens from shrimp, mite and CR show