Background: Hidden allergens in foods can induce allergic reactions. Currently it is not possible to estimate the exact prevalence of these reactions but they are clearly a growing problem.

Objective: The purpose of this study was to provide an overview of the role of hidden allergens (their importance, the types of food involved, and the severity of reactions) in allergic reactions in our geographical area.

Methods: A retrospective study was carried out in an adult population. Over a five year period, a total of 530 food reactions were reviewed.

Results: One hundred nineteen reactions (22.4%) were considered to be due to hidden allergens. Thirty-two percent of these were anaphylactic reactions. The most common hidden allergen was the Anisakis simplex larvae present in fish and shellfish. Fish allergens hidden in other foods caused reactions in 35% of fish-allergic patients. Twenty-two per cent of allergic reactions caused by eggs were due to egg allergens hidden in foods. All but one of the reactions caused by hidden legume allergens occurred in soy-allergic patients. Reactions caused by hidden fruits and hidden nuts were very uncommon.

Conclusion: Hidden allergens were the cause of a quarter of all food allergic reactions, mainly as a result of contamination and carelessness on the part of the patient. A simplex was the hidden allergen most frequently involved. Fruits and nuts were not frequent hidden allergens in our area. Reactions due to other uncommon foods such as soy, mustard, fl vourings, and honey were multiple and usually went unnoticed.
Only if the sources of hidden allergens are determined will it be possible to avoid such substances and thus guarantee the safety of the allergic patient.

Key words: Food allergy. Hidden allergens. Anaphylaxis. Foods. Anisakis simplex. Fruits. Nuts. Fish. Shellfish. Soy.