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If you introduce infant formula to your baby, you'll probably want to be on the alert for signs of an allergy, especially if allergies run in your--or the baby's father's--family. Most infant formula includes cow's milk or soy products in the ingredients. In cases of infant formula allergies, cow's milk protein is the most common allergen. Still, allergies to formula are not all that common; according to the U.S. Department of Agriculture, less than 3% of babies in the United States have an allergy to milk protein. Since half of the babies who are sensitive to cow's milk protein also turn out to react badly to soy, soy-based formula is not necessarily the solution if your baby does turn out to have a dairy allergy. Check with your baby's physician before switching to a hypoallegenic formula. It may reassure you to know that there's good chance your baby will outgrow an allergy to infant formula within a few years.