Why Severe Allergies Can Suddenly Pop Up in Adulthood

It's important first to understand why our body freaks out over the things we're allergic to. After all, there is nothing inherently dangerous about a little bee venom, just like a lobster roll and a peanut butter sandwich are, in and of themselves, pretty innocuous. It's the body's immune response to them — in the case of life-threatening allergic response, it's called anaphylaxis — that can be dangerous. Where one person's immune system can accept a walnut as just another source of nutrients, another's can decide that it's an ultra-threatening foreign invader. The immune system is staggeringly complex, and the reason it takes such resolute issue with certain substances is something of a mystery to scientists.

An anaphylactic reaction happens when your immune system comes across a protein it doesn't quite recognize and goes all code-red about it, causing specialized white blood cells called mast cells to explode, flooding the body with chemicals. The two most well-known of these are tryptase and histamine (which is why you take an antihistamine to fend off allergic reactions), both of which are unleashed on your body to help you survive an attack, but since there's no real enemy to dispatch, they can instead interfere with your blood pressure, cause your airways to swell, upset your stomach, and so on. This reaction occasionally can result in death.

Allergies — particularly food allergies — are more common in children, but research suggests adult-onset allergies are more common than previously thought.

"Allergies affect up to 30 percent of adults," says Dr. Tania Elliott, a spokesperson for the American College of Allergy, Asthma and Immunology. "While it is more common in childhood, we tend to see two peaks: first in school-age children, and then another peak around age 17-35. And food allergies are more common in children — approximately 8 percent versus 2 to 4 percent of adults. However, shellfish allergy is more common in adults."

But allergies arise in people — adults and children — who weren't allergic before simply because once your body is familiar with something, the body remembers. The immune system is famous for holding grudges, and its memory is long. For instance, being stung by a bee once is all that's required to put the immune system on alert. It might decide it's no big deal and let the whole thing slide, or it may decide it didn't like what it saw of that bee's venom and take note that it might decide to freak out a next time it encounters it.

"And there is no way to predict whether that next reaction is going to be life-threatening," says Elliot. "In fact, even if you have had a large reaction in the past, it doesn't mean it will get worse and worse each time. Now, the more times you are exposed to a bee sting, the more likely you are to have a reaction based off of sheer probability, but not because your immune system is any more primed to have a reaction."

In order, then, to have an allergy to something, your immune system has to be familiar with it. Once you've been exposed and it's flagged crabmeat or pistachios as a hazard, you'll be allergic to it for life.

"For food allergies, once you have a reaction, you are more at risk to have another, more severe reaction — even with trace amounts of the food," says Elliot. "If you have an allergy to one thing in the environment, and it doesn't get treated, you are more likely to develop an allergy to a second thing in the environment. Also, allergies tend to progress over time, so the first season it may just be itchy eyes, the next season, runny nose, the next, sinus issues and more infections, and even asthma."

The good news is, there are forms of immunotherapy that can help with allergies to insect venom (bees, yellow jackets, hornets and fire ants) and environmental aeroallergens such as trees, grasses, weeds and dust mites.

"We are making great strides for immunotherapies for food as well," says Elliot.