Egg allergies are caused by the same problem all allergies are caused by: an immune system malfunction. Your body misidentifies certain egg proteins as harmful even though they're perfectly safe. When you eat eggs, your body thinks it's being attacked and sends out an antibody called immunoglobulin E to fight off the proteins, or allergens. The antibody then triggers a number of chemicals, including histamine, to neutralize the proteins. Histamine is responsible for most allergic symptoms, including sneezing, hives and diarrhea.
There's no roadmap to explain why some people have egg allergies and others don't, but there are some factors that make you more likely to develop an egg allergy. First, family history plays a big part. If either or both of your parents has a food allergy or even another kind of allergy or asthma, your chances of being allergic to something are higher. Also, if you already have some kind of allergy, you're more likely to develop other allergies. Kids who have atopic dermatitis, which is a type of eczema, are more likely to have a food allergy than kids without. Age is also a factor: Children are more susceptible to egg allergies than adults. As they grow, their immune systems develop and they learn how to handle previously allergenic proteins.
Unfortunately, until your child outgrows his egg allergy, there isn't much you can do about it. He has to stay away from eggs and all egg-containing products in order to prevent allergic reactions. This means being aware of the obvious omelets and scrambled eggs, but it also means reading ingredient lists and looking for warning lines that indicate a product contains eggs or may have been in contact with eggs.