Just as filling a young child's ears with classical music may stimulate supreme intellect, exposing a kid to popular allergens early on may save him or her from debilitating allergies later.

Until recently, it was thought that the best way to prevent allergies was to keep the culprits — peanuts, pet dander, even intestinal worms — far away from children. But recent research may bring the peanut butter and jelly sandwich back to the table. Exposing kids to allergy triggers may also train their immune systems to recognize a genuine threat (serious illness) while disregarding minor irritants (the neighbor's dog).