An allergy is described as an overreaction of the immune system to foreign matter which actually poses no threat to the body. Rather harmless foreign matter (for example, the protein in cow's milk) is classed by the body as being dangerous. The immune system creates specific antibodies to protect against this apparent foreign matter. This therefore becomes an allergy trigger.

There are many inflammatory responses that trigger physical problems in those affected. Symptoms range from nausea, vomiting, diarrhoea, circulatory collapse, asthma, inflammation of the eyes and nasal mucous membranes, itching and redness of the skin, to atopic eczema and inflammations. The reaction is not triggered by whole foodstuffs, but by the protein components.

The degree of severity varies by allergen. Nut and peanut allergies can produce strong allergic reactions even in infancy.