Food allergies are less common than we tend to think. Nearly 20 percent of U.S. adults believe they have a food allergy, but when researchers looked closely at the symptoms people described, the actual number is only about half that.

Food allergies are often confused with food intolerances. A food allergy occurs when the body’s natural defenses, exposed to a certain food, overreact and treat it as an intruder, sending out chemicals to protect the body from the food. The reaction is serious and, in the case of anaphylactic shock, can be deadly. People with food allergies often carry epinephrine pens to counteract allergic reactions that make it difficult to breathe.

Close to half of the adults with food allergies had developed at least one of them in adulthood.

A food intolerance, on the other hand, is far less serious. It is usually a difficulty digesting certain foods and can be treated simply by avoiding the problematic food. Lactose intolerance that occurs when a person lacks the enzyme to properly digest milk is one example.

A national survey of over 40,000 adults found that while roughly one out of five people thought they had a food allergy, only about one in 10 actually had symptoms consistent with a food allergy. The other people had symptoms that were probably from a food intolerance or other food-related condition.

Researchers at Lurie Children’s Hospital of Chicago and Northwestern University found that only half of those with true food allergy symptoms had a diagnosis from a physician, and less than a quarter of those said they had a current prescription for epinephrine.

The most common food allergens among adults according to the study are shellfish, milk, peanut, tree nut, fin fish like salmon or tuna, egg, wheat, soy and sesame.

A surprising finding was that almost half of adults with food allergies developed at least one as an adult, something that is not considered typical. The top food allergen, shellfish, is a common food allergy across the lifespan and often begins in adulthood, according to Ruchi Gupta, lead author of the study.

More research is needed to determine why adult-onset food allergies develop and how they might be prevented, but if you suspect you have a food allergy, see a doctor for testing and proper diagnosis before eliminating foods from your diet. If you test positive for a food allergy, learn how to avoid the food in all its forms, and keep a current epi pen on you at all times.

The study is published in JAMA Network Open.