The main food crops have been established for about ten thousand years by selection of both crop genes and people who can tolerate the crops. Plants and animals have carried out biological warfare since they originated. Plants have to avoid being grazed out of existence and achieve that by devices including toxins, allergens, spines and shells. Crop plants are protected by their cultivators and their allergens and toxins have been eliminated by thousands of years of careful selection. Genetic engineering is starting to reintroduce the genes earlier eliminated by crop selection.
Any crop can cause food allergy but a few food items cause most food allergy.
These include peanuts, soybeans, wheat, tree nuts, crustaceans, fish, milk, and eggs. The allergens are the substances that promote the allergic response by the IgE components of the immune system. Most allergens are proteins while those related to drug allergy are 'haptens,' or small molecules associated with proteins. Traces of allergen may trigger a response in sensitive people but most allergy requires a fairly large intake. A response may take place minutes to hours after eating an offending food.