Egg allergies are most common among children; the good news is that most kids outgrow their egg allergies by the time they're five years old. Kids with egg allergies have confused immune systems: Instead of recognizing certain egg proteins as harmless, they think that those proteins are dangerous. Their bodies react to the proteins by releasing an antibody called immunoglobulin E, which then sends out a bunch of chemicals to fight off what have been flagged as harmful invaders. Those chemicals -- particularly the chemical histamine -- are what cause allergic symptoms.
Most children react to egg allergies with some sort of skin symptom: hives, rash, eczema or swelling. Other kids end up with gastrointestinal symptoms: stomachache, nausea, vomiting or diarrhea. Some children with egg allergies suffer respiratory symptoms that include a runny nose, sneezing, coughing or itchy, watery eyes. More rarely, children react to eggs with anaphylaxis. You can recognize the onset of anaphylaxis by noticing signs of a constricted airway that impedes breathing, a drop in blood pressure, an increased pulse, dizziness or even loss of consciousness. If your child seems to be in anaphylactic shock, he needs immediate medical care. If you have an epinephrine injection to give him, that's the time. The emergency room is your next stop.
If you think you've noticed some signs of an egg allergy in your child, you can ask your doctor to diagnose the situation. He'll usually refer you to an allergist who will ask about your family history of allergies and your child's history of symptoms. Then he might perform a skin test or a blood test to ascertain whether your child is allergic to eggs. Some doctors give a "food challenge" to kids in which they feed them a bit of the suspected food and then monitor them to see whether they react.