There are many types of things that can cause an allergy. Some folks are extremely sensitive to aeromatic petro-chemicals, pollution, and other things which no one should breathe, but usually doesn't bother most people too much. Probably most people have some food allergies but are not aware of it because it doesn't bother them much.
Some food allergies can cause severe and life-threatening reactions.
None of the food allergy recommendations here should be considered in the event that you have severe food allergies. Only mild food allergies and other types of allergies are discussed here. The IgE causes your body to release histamines which cause the allergy symptoms to appear. Since the IgE and histamines are cumulative and take several days for your body to get rid of, your allergy symptons will be much more severe with continued exposure. This is also the reason why your symptons will go away some days or only appear during certain seasons.