How Food Allergies Develop
While it's possible for any food to trigger a reaction, 80 to 90 percent of food allergies in this country are caused by eight major foods: milk, eggs, soy, wheat, peanuts, tree nuts (walnuts, cashews, almonds), fish, and shellfish (shrimp, crab, lobster). "Allergies to milk and eggs are the most common ones in babies and toddlers, followed by peanuts, soy, and wheat," says Robert Wood, MD, director of pediatric allergy and immunology at Johns Hopkins Children's Center, in Baltimore.
Doctors have been saying for years that kids usually outgrow most of these allergies by first grade, but allergies to peanuts, tree nuts, fish, and shellfish are lifelong. However, one recent study found that most kids actually remain allergic to milk and eggs until their teen years. And a 2007 Johns Hopkins study found that 79 percent of children had outgrown their peanut allergies by 16. Fish and shellfish allergies often don't develop until adolescence or adulthood, perhaps because kids usually don't eat these foods early in life. Doctors don't know yet whether kids will outgrow their allergy to sesame.
An allergic child won't have symptoms the first time he's exposed to a food, but his immune system secretly starts producing antibodies to fight the supposed invader. This can happen during breastfeeding, since a small amount of everything a mom eats is passed to her baby through her milk. In the future, when the child is exposed to that food -- either through breast milk or by eating it -- the antibodies release chemicals that trigger an allergic reaction. Symptoms can include skin flushing, itching, rashes, hives, swelling, vomiting, diarrhea, watery eyes, a runny nose, sneezing, and wheezing. Rarely, a food allergy can cause a life-threatening reaction known as anaphylaxis, in which the airways swell, blood pressure drops, and breathing shuts down.