Peanut allergies typically develop during childhood, and children less commonly outgrow them than they do other food allergies. Peanut allergy appears to be equally prevalent among American adults and children.

It has been well established that kids develop allergies to the “top eight” foods: milk, egg, wheat, soy, peanut, tree nuts, fish and shellfish. Most of the reactions in adults occur to the top eight foods as well, according to Dr. Sharon Chinthrajah, an assistant professor at the Stanford University School of Medicine and medical director of the Sean N. Parker Center for Allergy and Asthma Research, which was involved in shaping the survey. That said, shellfish allergy is more common among adults than among children, as it tends to appear later in life.

Several patterns have been observed that are unique to adults who develop new food allergies. One is called oral allergy syndrome, which occurs in a small percentage of adults who have seasonal allergies. It “involves your body getting tricked,” said Dr. Chinthrajah. She explained that some adults might have allergies to tree pollen, for example, and some of the tree proteins are similar to those in fruits and vegetables, “so when your body eats the raw form of those foods, it thinks you’re eating tree pollen.” Birch tree pollen, for instance, bears similarities to proteins found in fruits like peaches, apples and cherries. The main symptom is typically an itchy mouth or throat. Interestingly, if the fruit is first processed or cooked in any way, it denatures the protein and does not produce the same reaction.

Many adults who develop a new food allergy wonder what caused it — the “turn-on switch” as Dr. Gupta calls it. Anecdotal reports suggest that pregnancy, for example, can trigger new allergies, leading some to hypothesize that a hormonal connection may be at play. Other patients report they noticed a new allergy after getting a viral infection. Still, it is not yet clear what causes a new reaction to a food after someone has eaten it for decades without incident.

Importantly, an allergic reaction is not the same as a food intolerance. An allergic reaction is characterized by marked symptoms, such as itching, hives, swelling, trouble breathing or vomiting, within two hours of consuming the food in question. Symptoms that appear the next day may be characteristic of a food intolerance, which Dr. Chinthrajah said researchers do not yet understand as well as they understand food allergies.