(CBS) A 15-year-old girl from Canada, Christina Desforges, died last week after kissing her boyfriend. He had eaten a peanut butter sandwich nine hours earlier and she was allergic to peanuts.

Having an allergic reaction from kissing is actually not uncommon, The Early Show medical correspondent Dr. Emily Senay explains. A study of nut allergies a few years ago found about 5 percent of people in the study had an allergic reaction through kissing.

If you have a food allergy it can happen easily. A food allergy is when the body's immune system reacts against a specific component in a food that is not normally harmful to the body.

It doesn't take very much at all to provoke a reaction from the immune system, so even a tiny trace of food on a person's lips or mouth could be risky. Peanut allergies specifically can be easily triggered.

About 1.5 million people in the United States are severely allergic to peanuts and 50 to 100 people die every year because of a peanut allergy.

Common symptoms of a food allergy include flushing, wheezing, hives or a skin rash.

More severe reactions can include vomiting, difficulty breathing, a drop in blood pressure, loss of consciousness, and even a very severe reaction called anaphylaxis or anaphylactic shock, which can result in death. That's what happened to Desforges.

Many people with peanut allergies do tend to have severe cases, but you can have an allergic reaction from a kiss with any severe food allergy.

Peanuts and shellfish are very common allergies. Other foods that make up most food allergies are: milk, eggs, wheat, soy, and other tree nuts such as walnuts, pecans and almonds.

The best way to avoid a food allergy is to avoid the food that causes it. So when it comes to kissing, you should remind loved ones to wash their faces and brush their teeth if they know they've been eating something you're allergic to. And don't forget to read ingredients — a food you think is perfectly safe might not be.

Desforges was given a shot of adrenaline almost immediately, but it didn't save her. That's an unusually severe reaction, but it can happen.

If you think you have a food allergy but aren't sure, talk to a doctor who can help you be prepared by having an EpiPen on hand or other medications in case of an emergency.