• allergen matrix;
  • fish;
  • hidden food allergens;
  • Norwegian National Register for Severe Food Allergy Reactions;
  • shellfish

Hidden allergens in processed foods represent a health risk (1). About 2–3% of all adults and 6–8% of children are affected. About five times as many have experienced allergic symptoms after food intake at least once (2). In a case from the Norwegian National Register for Severe Food Allergy Reactions (MAR), a patient experienced an anaphylactic incident after having eaten a particular brand of fish cake. According to the ingredients list, it contained only fish (20% catfish), milk and vegetable proteins, components which had been inoffensive in the patient's medical history.

The patient's serum was tested against 12 allergens with the UniCAP® System (Pharmacia Diagnostics, Uppsala, Sweden), and on a matrix of 150 allergens, developed for the detailed specification of immunoglobulins (IgEs). All signals higher than twice the variance were evaluated, employing an empirical threshold value (0.05). Serum from a patient with a known shellfish allergy and pooled serum from healthy volunteers were used as controls. The fish cake sample, protein extracts from several fishes and shellfishes, and purified cod parvalbumin and shrimp tropomyosin were analysed by dot and Western blots, using the sera.

With UniCAP®, the patient had specific IgE class 2 (0.7–<3.5 kUA/l) reaction against four allergens not related to this case. On the allergen matrix, the serum reacted against lobster (0.39) and not against fish proteins, but weakly against cod (0.07). The positive control serum elicited signals to shrimp (5.24), lobster (9.66), crab (2.89), crawfish (10.42) and squid (0.346). The negative control reacted weakly to some fishes but not to shellfishes. On Western blots, the patient's serum reacted against shrimp (36 kDa, 20 kDa), lobster (37 kDa, 20 kDa), tropomyosin (36 kDa) and cod (45 kDa), but not against catfish, salmon and parvalbumin. A protein of a molecular weight (35 kDa) similar to the major shellfish allergen tropomyosin was recognized in the fish cake extract, confirmed by the positive control serum and not detected by the negative control.

Food allergy is one major form of adverse reaction to foods (3), and about 200 ingredients are confirmed as causative agents. Alert systems (4) and correct food labelling are therefore actual issues with the Food Authorities. In this case, elevated anti-lobster IgE and low anti-cod IgE were found by our sensitive allergen test matrix, whereas they were below the quantification limit for UniCAP® (<0.35 kUA/l). The results of the blot experiments can be explained by cross reactions between similar epitopes in the Crustacea tropomyosins (5), a pan-allergen group which causes 80% of all shellfish incidents. The identified 45 kDa cod protein hints at a monospecific cod allergy (6). Our study encouraged the manufacturer of the fish cake to intensify the washing between different product batches, as the hidden allergen could be tracked down to cross-contamination by a shellfish pastry produced on the same manufacturing line.

The authors thank Berit Stensby for technical assistance and Ivar Fæste for contributing raw material. This study was financially supported by the National Food Safety Authority and the Research Council of Norway.


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