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An allergic reaction to a food is an unpleasant reaction caused by the immune system overreacting to a food. The most common type of food allergy is caused by an overly sensitive immune system that creates IgE antibodies directed against an otherwise harmless protein in the food (allergens).

Unpleasant reactions to foods not caused by the immune system are considered to be food intolerances. Food intolerances have various causes. For example, a person may lack an enzyme needed to digest a portion of a certain food (e.g. lactose intolerance) or the food may be contaminated by bacterial or other toxins that cause symptoms resembling those of food allergy (e.g. food poisoning).