Diagnosis of peanut allergy has improved thanks to component-resolved diagnostics. Peanut allergen component Ara h 2 is considered to indicate true peanut allergy. The component Ara h 6 is structurally similar to Ara h 2, but the diagnostic value of analyzing IgE antibodies to Ara h 6 is unclear. A boy sensitized (≥0.35 kUA/l) to Ara h 8 but not to Ara h 1, Ara h 2 and Ara h 3 was challenged with peanut and developed grade II anaphylaxis. In serum collected at the time of challenge a doubling of IgE to the peanut allergen extract was observed compared to allergy testing 9 months earlier. In contrast, IgE levels to Ara h 1, Ara h 2, Ara h 3 and to Ara h 8 were rather unchanged. After another 2 months, Ara h 6 was analyzed and revealed a level of 24 kUA/l whilst Ara h 2 was 0.12 kUA/l. We suggest that IgE sensitization to Ara h 6 caused the reaction and conclude that analyses of IgE levels to peanut and peanut components should be performed in connection with a challenge. Furthermore, levels to Ara h 2 below 0.35 kUA/l may still indicate a risk of severe reaction at the time of challenge since in rare cases, Ara h 6 IgE antibodies may be present without occurrence of IgE antibodies to Ara h 2.

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