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If you are part of the approximately 4 percent of adult Americans who suffer from a food allergy, you might be interested to know why the peanut butter on a sandwich could kill you with one bite, while the jam is harmful only to your waistline.

All allergic reactions , basically, are the result of the body's defense mechanism against foreign intruders it (mistakenly) perceives as harmful. Your immune system responds to the threat by pumping out antibodies, which in turn trigger the release of protective chemicals that make your nose runny, eyes itchy and, sometimes, make it impossible to breathe.

Though any food can cause such an allergic response, some are notorious for provoking the immune system, while others are almost universally innocuous to the human race. Nuts, seeds and shellfish are well-known allergens, as are corn, milk, soy, eggs and wheat. In fact, these account for 90 percent of all food allergies in America, according the U.S. Food and Drug Administration.

Although it may seem that the muscle of a shrimp, the bean of a plant, and the milk of a mammal are very different substances, the unifying thread of the "big eight" allergens is the type of protein they contain.

"It is fairly clear that these do not have much in common," said Scott Sicherer, a Professor of Pediatrics at the Jaffe Food Allergy Institute at Mount Sinai in New York. "What they do seem to have in common is that the proteins in them, to which allergy occurs, are relatively stable to digestion, presumably allowing the immune system to see them more easily."

When the proteins in question are spotted by the immune system in people with allergies, the type of antibody put into action is the whimsically named Immunoglobulin E (IgE), which humans first developed as a defense against parasites, scientists believe.

The IgE make-up is different in every individual, and people with allergies will typically have more of these antibodies. The tendency for IgE to attack a harmless substance is hereditary, so "confused" IgE antibodies are passed on. That may be why children with allergy-prone parents are more likely to develop allergies themselves, though not necessarily to the same things.