Q. I’m 32 and just had my first anaphylaxis experience to shrimp. It was scary: I was wheezing and could hardly breathe. How can a grown woman suddenly develop a food allergy?
Dr. Scott Sicherer: You are not alone in developing shrimp allergy as an adult. In our U.S. prevalence studies and studies in Canada that tracked peanut, tree nut, fish, shellfish  and sesame allergies, shellfish was the most common self-reported allergy.
We know food allergies are affected by the immune system, heredity, environmental factors, and the characteristics of the foods themselves. We know that many food allergies are outgrown, so there are clearly differences between children and adults. What we are missing are the exact details underlying each factor and how the factors interrelate. We only have theories to address your question.
The characteristics of food proteins likely play a role. Proteins responsible for persistent and severe allergies are more resistant to digestion and more likely to be recognized by the immune system. It may be that the child’s immature gut or immune system is more prone to attack the proteins, but shrimp may be an example of a protein that is particularly capable of triggering an immune attack even for adults.
A route of exposure other than through the mouth may be a contributing problem for adult-onset allergy. Most food allergies that begin beyond childhood are mild reactions to raw fruits and vegetables. This type of allergy is called oral allergy syndrome and is related to proteins in pollens. For example, apple shares similar proteins with birch pollen. Thus, becoming allergic to similar proteins in the air starts the trouble.
A theory has also been proposed that environmental exposure to peanut, without actually eating peanut, may increase the risk of peanut allergy. Interestingly, shellfish proteins are similar to ones found in dust mite and cockroach, although most people with those allergies tolerate shellfish.
Next: Digestion Changes
Digestion changes might also play a role. There are cases where adults became allergic to a food they previously tolerated after taking antacids. This theory states that the lack of stomach acid reduces digestion and allows the protein to pass to the immune system intact. This probably does not account for many cases of allergy because antacids are widely used by those who never develop problems, but it raises a possibility that some change in digestion may be a factor.
Another possibility is that some insult against the immune system, for example a viral illness, might trigger an imbalance, leading to a new attack on an innocent food protein.
The problem in figuring out why a new allergy suddenly develops is that there are numerous possibilities and nothing has so far stood out as a clear reason. The long list of possibilities, well beyond the examples I have given, presents an active area for research.
It’s important to discuss your allergy with a board-certified allergist, and to consider any circumstances that might have differed at the time of the reaction, such as exercise, alcohol or medication use, which sometimes play a role in precipitating reactions. It is important to ensure you have the correct diagnosis, learn how to successfully avoid the trigger food(s), and know what to do in the event of a future allergic reaction.
Dr. Scott Sicherer is Chief of the Division of Allergy and Immunology of the Jaffe Food Allergy Institute at the Icahn School of Medicine at Mount Sinai in New York. Together with Dr. Hemant Sharma, Associate Chief of the Division of Allergy and Immunology at Children’s National Medical Center in Washington, he writes “The Food Allergy Experts” column in the American Edition of Allergic Living magazine. Questions submitted below will be considered for answer in the magazine.