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).
Food or pollen. Some allergists suggest later start for baby foods, rather than starting at 3 or 4 months of age. Milk, wheat, and eggs are some of the more common foods to cause allergies. Pollen is a problem at certain times of the year. Trees in the spring, grass in the summer, and weeds in the fall. The specific ones will depend on where you live. An over-the-counter antihistamine may help.