Allergy is a catch-all word for a wide variety of reactions to substances that the body determines to be foreign. Anything that it does not recognize as itself can be considered an allergen.
Substances in foods, house dust and pollen can cause anything from sniffles to migraine and even sudden death. Allergies are becoming increasingly common, but they are still not fully understood.
Almost 12 million people have food allergies annually and the prevalence is rising in the contemporary world.