Why do some children have allergies?

Hereditary: The ability to have allergies is hereditary. If one parent has allergy, there is about a 40% chance of the child having allergy. If both parents are allergic, the chances increase to about 70%. Allergy also requires exposure to an "allergen", like pollen, dust mite, molds or animals. So if there is no exposure, no symptoms occur.

Genes + environment: To develop an allergy, you need a genetic predisposition. For example, children at highest risk of developing allergic asthma are those whose parents have asthma. Also necessary is multiple exposures to the allergen. The strongest predictor of developing allergies in the future is having allergic disease now (for example, a child with eczema has increased chance of developing asthma).

Exposure + Genes: One needs both a genetic component and "exposure" to a said allergen to develop an allergy. There is a growing support over the past 20 years, that growing up in an environment which is "too clean" can also lead to development of allergies down the road. Either way, allergies are on the rise.