For example, a soybean engineered to contain genes from a brazil nut was found to produce allergic reactions in blood serum of individuals with nut allergies. Allergic reactions to nuts can be serious and even fatal. Researchers were able to identify the danger in this particular case because nut allergies are common and it was possible to conduct proper tests on blood serum from allergic individuals. In other cases, testing for allergenic potential can be much more difficult. When genetic engineering causes a familiar food to start producing a substance previously not present in the human food supply, it is impossible to know who may have an allergic reaction.
Genetic engineering has the potential to make ordinary, familiar foods become toxic.
In some cases, new characteristics introduced intentionally may create toxicity. The Bt toxin as it appears in the bacteria that produce it naturally is considered relatively safe for humans. Humans have little experience with exposure to this form of the toxin. Furthermore, in the past humans have had no opportunity or reason to ingest any form of the Bt toxin in large quantities. And of course, a pesticide engineered into every cell of a food source cannot simply be washed off before a meal.