An egg allergy is rare in adults and most often seen in children from 2 to 7 years of age. Studies show that most children tend to outgrow an egg allergy within a few years. However, if the egg allergy is severe, it could last a lifetime. An egg allergy is caused by a protein in eggs which triggers an abnormal reaction to the immune system. Eggs have two allergenic components: the yolk and the white. An allergic reaction can vary from mild to life-threatening, depending on the person and the amount of egg consumed.