A food allergy is a hypersensitive reaction to a normally harmless substance in food and is quite common in babies and children. Some children are more likely to have an allergic reaction triggered by a particular food. Those most at risk are babies and children from families with a history of allergies. By the time a child is 2 years old they would have more than likely grown out of their allergy. However other children will have a food allergy throughout their life.