Foods can induce a variety of reactions, some of which represent true allergic phenomena produced by an immune mechanism, while others are intolerance reactions.

Some foodstuffs can produce rapid allergic reactions when ingested by sensitised individuals. The manifestations in such cases can range from urticaria or angioedema to potentially fatal anaphylactic reactions. This is the case of foods such as shellfish, fruit, eggs and milk.

Other foodstuffs produce more delayed allergic reactions because they require prior digestion. The symptoms in such situations mainly affect the skin.

Milk and eggs are the foods most often associated to such reactions in patients with dermatitis and digestive symptoms, while fish is the food most commonly linked to asthma. Certain nuts and fresh fruits can lead to itching in the oral cavity.

In adults, the foodstuffs most commonly associated to allergy are fruits, nuts, vegetables, fish and shellfish.

In children, sensitisation most frequently develops in response to cow's milk and eggs, with fish ranking a distant third. After the age of 5 years, adult-type allergies appear. Unlike adult and other types of allergy, most children who suffer foodstuff allergies eventually stop presenting these manifestations with time.