Egg allergies can cause a severe reaction called anaphylaxis. Anaphylaxis may begin with some of the same symptoms as a less severe reaction, but then quickly worsen. A person might have trouble breathing, feel lightheaded, or pass out. If it's not treated, anaphylaxis can be life threatening.
Egg allergy usually first shows up when kids are very young. Most kids outgrow an egg allergy by the time they're 5 years old, but some people stay allergic.