Q:

I'm in my mid-30s and I've never had food allergies. But lately I've been getting symptoms that seem like an allergic reaction after I eat shellfish -- itchiness and swollen lips. I thought food allergies appeared in childhood or not at all, but I'm starting to question this. Is it possible for me to develop a food allergy as an adult?

A:

Contrary to what you (and many others) assume about food allergies, they can actually crop up without warning at any time of life. So even if you didn't have a food allergy during childhood, you're not necessarily off the hook.

That said, food allergies don't rank high on the list of later-life maladies. Only 4 percent of adults are allergic to a food, and even youngsters who have the most common food allergies -- to milk, eggs, wheat and soy -- are likely to outgrow them by the time they enter kindergarten.

Yet no adult is truly immune to food allergy. Some food allergies -- especially to peanuts, tree nuts (which include almonds, cashews and walnuts), fish and shellfish -- persist from childhood. But adults can also be waylaid by allergic reactions to foods they've enjoyed all their lives. Moreover, a food allergy that first rears its head in adulthood isn't likely to go away.

Experts have two explanations for food allergies that develop in adulthood. They may be the result of a delayed or extended period of sensitization to an allergen, or they may reflect a cross-reaction to some other allergen, such as pollen.

Not all bad reactions to food are food allergies. Lactose intolerance, for example, is not an allergic reaction, but rather an inability to digest the sugar in milk and other dairy products because of a deficiency of lactase, the enzyme that breaks down milk sugar (lactose) so it can be digested.

Food allergies represent a failure of the collaboration between the digestive and the immune systems. When you eat, your digestive system breaks down the food into its molecular components. The first time you digest a food, your immune system examines the food's array of molecules. If your immune system decides that these molecules do not pose a threat, it gives them the equivalent of a passport to your body through a process known as oral tolerance.

A food you're allergic to gets rougher treatment. For some unknown reason, the immune system misidentifies a food molecule as harmful and initiates an offensive against it. Molecules that provoke such reactions are called allergens.

The immune system's two principal players in an allergic reaction are IgE antibodies and mast cells, which cluster in tissues that line the portals to the body -- the skin, airways and digestive system. IgE antibodies are produced to snare a particular allergen. They dock onto receptors on mast cells where they lie in wait for the allergen to appear -- a process known as sensitization.

Once sensitized, the immune system is ready to pounce next time the allergen shows up. When an IgE antibody that is docked on a mast cell snags an allergen, the cell releases histamine and other compounds. The surge of chemicals can cause a variety of reactions, including sneezing, a runny nose, itchy eyes, rashes, hives and wheezing.

If histamines are released throughout the body, more serious problems may develop. The airways may constrict. Nausea, vomiting and diarrhea can follow. And blood pressure may plummet, leading to loss of consciousness and the uncommon but life-threatening situation known as anaphylaxis.

You usually know you're allergic to a food within minutes of eating it. The reaction may range from a mild response -- such as itching or swelling around the mouth -- to anaphylaxis. But it's also unpredictable: A person who is allergic to a food may have a mild reaction one time and full-blown anaphylaxis the next.

Fish and shellfish allergies are the most common serious adult-onset food allergies. Some researchers speculate that fish allergies have a late onset because many people don't eat much fish until they grow up, so they become sensitized to it later.

Others scientists who study cross-reactivity between fish and other allergens have found that people who are allergic to lobster, shrimp and other shellfish are also allergic to house mites and cockroaches. The principal suspected allergen is a protein called tropomyosin, which is shared by crustaceans, mollusks, roaches and mites.

If you have a food allergy, you'll have to plan your meals with care so you can avoid exposure to the culprit allergen. In your case, that means crossing shellfish off your menu. And people who are at risk for severe reactions should have an EpiPen with them so they can give themselves a shot of epinephrine to fight back against the reaction. It may seem complicated at first, but with a little care your meals will be both tasty and safe.

(Submit questions to harvard_adviser@hms.harvard.edu.)

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