Some studies have suggested that allergens may appear in the circulation after ingestion of allergenic food<br/>sources. The reported levels of allergen in serum, however, are low, and conclusions between studies differ.<br/>Here, we investigated factors that determine the detection of allergens in serum after consumption of peanuts.<br/>Ten healthy volunteers ingested 100 g of light-roasted peanuts. Serumsampleswere taken at regular intervals for<br/>six hours. A double monoclonal sandwich ELISAwas used to analyse the presence and quantity of the major peanut<br/>allergen Ara h 6 in serum.<br/>In 4 out of 10 subjects, no Ara h 6 could be detected. Purified Ara h 6 thatwas digested in vitrowas still reactive in<br/>the ELISA, rejecting the possibility that digestion leads to small peptides that could not be detected. Spiking of purified<br/>Ara h 6 in baseline serum showed that the pre-ingestion serum of these four subjects partially prevented<br/>Ara h 6 to react in the ELISA, with a reduction of reactivity of up to 3 orders of magnitude or more. Pre-ingestion<br/>serum of the other six subjects did not show such an effect. The reduction of reactivity of Ara h 6 coincided with<br/>high titres of IgG and IgG4, and removal of IgG from pre-ingestion serum abolished this effect completely, indicating<br/>that IgG and IgG4 inhibited the reactivity of Ara h6 in the ELISA.<br/>Weconclude that some individuals have IgG and IgG4 against food allergens in their blood, which interfereswith<br/>detection of such food allergens in serum. Because this effect does not occur for each individual, the possibility of<br/>such interference should be taken into considerationwhen interpreting immunochemical studies on the absorption<br/>of food allergens in serum.