Food allergies: Symptoms, causes, testing and treatment

How do allergies happen?
To be allergic to something, your immune system must become sensitized to it, says Waserman, who explains that blood contains antibodies (cells that are part of your immune system), which normally fight invading organisms such as bacteria or viruses. Among these is an antibody called IgE, whose role is to identify substances, such as parasites, that should be attacked. For unknown reasons, IgE sometimes becomes programmed to identify something else – such as a protein from a specific food – as a substance that needs to be destroyed.

IgE sits on special cells called mast cells, which are filled with different types of granules containing chemicals. "If you're allergic to peanuts and accidentally eat something containing them, IgE recognizes the peanut allergen and binds to it," explains Waserman. This causes the mast cell to burst apart and discharge its army of chemicals. These chemicals then cause the symptoms seen in an allergic reaction to food, including hives, itchy skin, vomiting, lip and tongue swelling, shortness of breath and even low blood pressure.

Since food allergens get into the blood, they can travel anywhere in the body. That is why, when you eat a food you are allergic to, hives can appear on any part of your body.

Food allergies versus food intolerances
It's important to distinguish a true food allergy from a food intolerance. This is because allergies have the potential to progress to something life-threatening. Food intolerances (or food sensitivities) can be very uncomfortable, but it's highly unlikely they will kill you.

Since people often don't know the difference between an intolerance and a true allergy, they may not get appropriate treatment. But the two are very different beasts, says Waserman. Food intolerances have a huge variety of causes, ranging from a lack of certain digestive enzymes (as in lactose intolerance), abnormalities in the structure of the bowel or intestines (as in irritable bowel syndrome) and inherited conditions; they rarely involve the immune system.

With a true allergy, an adverse reaction to a food occurs more quickly than one from an intolerance; symptoms can start within minutes or even seconds. A food allergy reaction often includes itchy skin and swelling of the throat.

If you develop any sort of reaction to a food, says Waserman, you'll need to get tested by an allergist to find out whether or not the problem is an allergy. Allergists commonly do skin tests, which involve placing a small drop of allergen on the skin and then pricking the skin through it to see if there is a reaction, such as a hive and surrounding redness. Another test is the oral food allergy challenge: In a medically controlled environment, you are given increasing amounts of suspect foods to see if and how you react to them.

Many different foods can cause allergic reactions, but the top foods people are allergic to are shellfish, fish, peanuts (a legume, not an actual nut), eggs, milk, tree nuts (which includes everything from almonds to pine nuts to walnuts), wheat, soy and sesame seeds.

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