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March 12, 2015

The science of everything: Allergies

Peanuts. Pollen. Pet dander. 

Such things aren't inherently dangerous to humans, yet if you have an allergy, your body reacts like it is being attacked by a diseased invader. Eyes water, skin itches, and mucus flows freely — all in an attempt to rid yourself of any trace of this relatively harmless substance. 
  • This is the latest installment in a series called “The Science of Everything,” an opportunity for science journalist Meeri Kim, Ph.D., to explore the how and why of everyday things.
Allergic reactions can range from annoying to debilitating, or even fatal. So what is the benefit of having our immune systems overreact in this way? Why have allergies evolved in humans? It seems entirely counterproductive, especially since the majority of the population can handle being around ragweed, mold and cats just fine. 

A few theories exist, but scientists still don't know for sure why allergies plague us. They do agree, however, that a hypersensitive immune system is to blame for the symptoms. In particular, Y-shaped proteins called antibodies produced by the body's immune system — normally responsible for detecting bacteria, viruses, and parasites — recognize and latch onto pollen and other foreign substances in order to remove them from the body. 

The type involved in allergic reactions is called immunoglobulin E (IgE), a class of antibodies ordinarily used to fight against parasites. After an initial exposure, the immune system produces tailor-made IgEs that sit atop mast cells located in the mucus membranes of our eyes, nose, lungs, and skin. Once an IgE binds to that specific pollen grain or dust particle, the allergy cascade is triggered.

“The mast cells basically explode and release their contents, and sitting in the mast cells are a number of things — the main one being histamine,” said otolaryngologist John H. Krouse of the Temple Allergy and Sinus Center. “That's the chemical that drives the acute response, the itching and sneezing.”

So what makes these antibodies — designed to help defend the body against hookworms and tapeworms — go rogue over peanuts and hamsters? 

Our genes are at least partially responsible. This means children can inherit allergic reactions from their parents, although specific allergies aren't passed down. One 2013 study from physicians at Johns Hopkins School of Medicine found a particular genetic mutation that can kick off the immune system chain reaction and strongly predispose a person to allergies. 

“You have to have the genetic predisposition to respond in an allergic way, and if you're not genetically programmed to respond, you won't,” said Krouse. “If you don't have that potential, then you can be exposed to tree pollen all day, and you're not going to have a response to it.”

While allergies are a common disorder, the innocuous substances that cause each of our immune systems to go into overdrive — called allergens — tend to vary from person to person. Pollen from trees and grasses cause many to sniffle and sneeze once warmer weather comes around. And even though pollen counts die down during the cold of winter, those with indoor allergies to dust and cockroaches can't catch a break.

Food allergies are more common in children than adults, with the top culprits being milk, soy, eggs, wheat and peanuts. Then there are allergies to drugs, latex, insect stings — along with more rare hypersensitivities to things like leather and oranges

While our parents may have given us the capacity to become allergic, specific allergies develop as a result of exposure. One common theory, called the hygiene hypothesis, suggests that the rise in allergy prevalence is a byproduct of our obsession with cleanliness

“Modern society is partly to blame,” said Eric J. Schenkel, medical director of the Valley Allergy & Asthma Treatment Center in Bethlehem, Pa. “If you look at where allergies are more severe right now, they are industrialized societies where we keep things very clean.”

In 2011, a study in the New England Journal of Medicine famously found that a group of children living on farms in Central Europe had lower rates of asthma and allergies than controls who did not live on farms. Children from farms were exposed to a wider range of environmental fungi and bacteria, which was thought to protect them from developing these disorders. 

“There's been a lot of data that shows if you get exposed to dirt or a farm at a young age, you'll have less allergy problems as you are older,” Schenkel said. “I wouldn't recommend rolling in the dirt or getting a cat to prevent allergies, but that's very provocative information.”

The idea is this: The immune system at birth is relatively naive, since it hasn't encountered invaders yet. But as soon as we are out in the pathogen-ridden world, entities called helper T cells develop into a variety of subtypes that fight against different enemies in targeted ways. For instance, Type 1 cells are primed to kill bacteria and viruses directly, whereas Type 2 cells work defensively by strengthening the body's protective barriers like mucus membrane secretion. 

While Type 2 helper T cells can successfully defend against parasites, they also can recognize an allergen as harmful and trigger the production of IgE antibodies to gear up for the next encounter. 

For many of us, parasites aren't a major problem, so most of our immune system's resources get pulled in the Type 1 direction. Then, later in life, our immune system ends up overreacting to otherwise harmless substances that resemble Type 2 pathogens. 

“Sometimes, the surface coating of a pollen granule may look like the surface coating of a parasite, and that's how you get allergies,” said Krouse. “The body is trying to protect against something that looks like something else.”

In other words, our immune systems may have trouble distinguishing the molecular markers of a pollen grain or speck of pet dander that happens to mimic another threatening substance. But like a puzzle, if the molecular pieces fit, an allergic reaction will be unlocked. 

So in the end, maybe allergies aren't meant to be beneficial or protective — they could just be a rotten case of mistaken identity.