Why Severe Allergies Can Appear Unexpectedly in Adulthood

Imagine you're a beekeeper. Part of your job involves the occasional sting. It doesn’t bother you much. But one day, while working, you get stung by a couple of bees — no big deal, you've been stung countless times before. However, after the sting, your lips begin swelling up like croissants, your tongue feels like it's made of stone, your throat starts tightening, and you feel faint with a frantic itchiness. You're rushed to the hospital, and it takes a hefty dose of epinephrine and an IV drip of who-knows-what to get you back to normal.

You survive, but now it turns out you're severely allergic to bees. A similar thing can happen to adults who suddenly find themselves experiencing a severe reaction to foods like shellfish, peanuts, or tree nuts. What’s happening here?

What Causes Allergies?

To understand why our body reacts so violently to things we're allergic to, it's important to recognize that there's nothing inherently dangerous about substances like bee venom, lobster rolls, or peanut butter sandwiches on their own. The issue lies in the body’s immune response — specifically, the life-threatening allergic reaction known as anaphylaxis. One person’s immune system might simply see a walnut as just another food, while another’s might treat it as a grave threat. The immune system is incredibly complex, and scientists still don’t fully understand why it overreacts to certain substances.

An anaphylactic reaction occurs when your immune system encounters a protein it doesn’t recognize, causing a full-on alarm. Specialized white blood cells, known as mast cells, burst open and release chemicals like tryptase and histamine (which is why antihistamines help combat allergies). These chemicals are intended to protect the body, but since there’s no real threat to fight off, they can cause your blood pressure to drop, your airways to swell, upset your stomach, and more. In extreme cases, this response can even lead to death.

Late Allergy Development

While allergies, particularly food allergies, are more common in children, research indicates that adult-onset allergies are becoming more widespread than we once thought.

"Up to 30 percent of adults experience allergies," states Dr. Tania Elliott, a spokesperson for the American College of Allergy, Asthma and Immunology. "Although more prevalent in children, we observe two main peaks: one in school-age kids and another between ages 17-35. While food allergies are more common in children — about 8 percent versus 2 to 4 percent in adults — shellfish allergies are notably more frequent in adults."

Allergies can develop in individuals — both adults and children — who weren’t previously allergic because once your body encounters something, it remembers. The immune system has a reputation for holding grudges, and its memory is long. For example, a single bee sting is enough to alert the immune system. It may either ignore it or decide that the venom is a threat and prepare for a stronger response the next time it encounters it.

"There’s no way to predict if the next reaction will be life-threatening," says Elliott. "Even if you’ve had a significant reaction before, it doesn’t guarantee it will get worse each time. The more often you’re exposed to something like a bee sting, the more likely a reaction will happen purely based on probability, but not because your immune system is more sensitive."

Once Allergic, Always Allergic

For an allergy to develop, your immune system has to first recognize the substance. Once you've encountered it and your body identifies crabmeat or pistachios as a threat, the allergy remains for life.

"Once you experience a food allergy reaction, you become more vulnerable to a potentially more severe reaction — even from tiny traces of the food," explains Elliot. "If you’re allergic to one environmental factor and it remains untreated, you're at a higher risk of developing an allergy to something else in your surroundings. Additionally, allergies tend to worsen over time. For example, the first season might just cause itchy eyes, but the next could bring a runny nose, followed by sinus problems, increased infections, and even asthma."

The good news is that immunotherapy treatments exist to help with allergies to insect venom (such as from bees, yellow jackets, hornets, and fire ants) and environmental allergens like trees, grasses, weeds, and dust mites.

"We’re also making great strides in immunotherapies for food allergies," says Elliot.