From childhood through her early 20s, Katherine Wu was severely allergic to cats, with exposure causing her throat to swell and her chest to "erupt in crimson hives." Then, one day, her allergies disappeared. Writing for The Atlantic, Wu highlights how experts are unsure about how and why some allergies resolve over time.

What are allergies and how do they occur?

According to Wu, allergies are essentially "molecular screwups" that occur when an immune system mistakes a harmless substance as dangerous and begins to attack it.

When it comes to most allergies, an allergen, such as almond or grass, will trigger certain immune cells, which then produce an antibody called IgE. IgE then triggers other defensive cells and molecules, which release inflammation-promoting signals, such as histamine.

This inflammatory response ultimately sparks "bouts of itching, redness, and swelling," Wu writes. "Blood vessels dilate; mucus floods out in gobs. At their most extreme, these reactions get so gnarly that they can kill."

Currently, around 50 million people in the United States experience some kind of allergy every year, which can impact their quality of life, access to certain treatments, and more.

Some allergies, particularly those that often appear in children, will go away over time. For example, around 60% to 80% of milk, wheat, and egg allergies will usually resolve by puberty. Roughly 80% to 90% of penicillin allergies typically go away within 10 years of initial detection.

However, other allergies, such as those toward peanuts, tree nuts, and animals like cats and dogs, are considered a "permanent diagnosis," since they are often difficult to neutralize. For example, the main cat allergen, a protein called "Fel d 1," can linger in an area up to six months even if the cat is no longer there.

Is there a way to get rid of allergies permanently?

According to Wu, "experts have a broad sense of how allergies play out in the body," but "far less is known about what causes them to come and go—an enigma that's becoming more worrying as rates of allergy continue to climb."

When someone becomes less allergic to a particular substance, this is called "tolerance." However, because allergies can manifest in different ways, there is no single way than an allergy can become less severe or disappear on its own.

For some people, their bodies just begin to make less IgE over time, making them less sensitive to allergens. Other people's immune systems may begin to produce another antibody called IgG4 that can counter IgE or a molecule called IL-10 that can stop immune cells from reacting to IgE.

Some people, like Wu, may see their allergies just disappear one day, although such situations are often considered an anomaly.

"We don't fully understand how these things go away," said Zachary Rubin, a pediatrician at Oak Brook Allergists in Illinois.

For people whose allergies continue to persist, allergists may try to "nudge the body toward tolerance," Wu writes. Some treatments include shots or mouth drops that slowly introduce small amounts of allergens into their systems over months or years, which Wu calls "the immunological version of exposure therapy."

Although these treatments can help improve some people's allergies, researchers are still not sure how the allergy shots help people develop tolerance, just that "they work well for a lot of patients," Rubin said.

According to Wu, part of the problem with treating allergies is that they "can involve just about every nook and cranny of the immune system," meaning "scientists have to repeatedly look at people's blood, gut, or airway to figure out what sorts of cells and molecules are lurking about, all while tracking their symptoms and exposures, which doesn't come easy or cheap."

Currently, researchers are studying how and why allergies sometimes resolve, whether over time or suddenly, to potentially engineer new treatments for those with allergies. For example, there are new antibody-based treatments that could prevent IgE from reacting to allergens.

Researchers are also testing if fecal transplants of gut microbiome from non-allergic people into those with allergies could help reduce certain food sensitivities. Some experts are encouraging parents to expose their children to sometimes-allergenic substances, like milk and peanuts, early on to potentially build tolerance.

Overall, "[a]llergy, like the rest of the immune system, is a hot, complicated mess—a common fixture of modern living that many of us take for granted, but that remains, in so many cases, a mystery," Wu writes. (Wu, The Atlantic, 10/5)