Since her son Matt was an infant, Lynda Mitchell knew he had a milk allergy. She controlled the symptoms by switching his baby formula. But when he was 1, Halloween proved horrifying. While trick-or-treating in a pumpkin costume, Matt grabbed a chocolate bar and bit through the wrapper. Although he spit out the candy, his violent allergic reaction morphed into a mess of hives, swelling and labored breathing. He ended up in the emergency room.