Why Do We Have Allergies?

Jun 14, 2018
Originally published on June 15, 2018 2:48 pm

Summer’s back and every plant wants to fertilize your nose. At least that’s what it feels like if you have allergies. Itchy eyes, runny nose, constant coughing and sneezing — pollen can make us miserable.

But why? Pollen’s not dangerous; it’s a fundamental part of life on land. For hundreds of millions of years, every land animal ever has lived with pollen. You’d think we’d have had plenty of time to evolve a way to deal with it. So how have our bodies not figured this out yet?

It helps to know a little bit about why pollen exists in the first place. Around 500 million years ago, plants began to live on land. The earliest land plants were relatives of today’s liverworts, mosses and ferns. Their male spores needed to swim and needed wet environments to successfully reproduce, said Andy Jones, a plant evolutionary biologist at Oregon State University.

The evolution of pollen opened up a new world for plants. By surrounding DNA with a protein coat, pollen could protect a plant’s genetic material from drying out and from the sun’s harmful UV radiation. Pollen can survive for days in the open air, allowing animals and the wind to carry it far and wide.

The pollen that causes most allergies comes from trees, grasses and weeds that rely on wind pollination. Pollen from flowers and plants that rely mainly on animals like bees, tends to be too heavy to cause many problems, Jones says. The trick to wind pollination is to create billions of spores. It just takes one grain to fertilize the female part of a plant, so casting fate to the wind isn’t a bad strategy.

“It’s remarkable that something so random actually ends up working,” Jones said.

For the hundreds of millions of years that plants have been relying on the wind for pollination, the landscape has been coated with excess pollen. Animals in that environment evolved with pollen all around them.

As far we know, for animals in the wild today, pollen doesn’t cause problems. “To my knowledge no wild animals have been observed allergy,” said Lars Hellman, an immunology researcher at Uppsala University in Sweden who has studied allergies in wild animals such as wolves. “Pollen is probably not dangerous for any animal.”

The problem isn’t that pollen is “evil,” said Ruslan Medzhitov, an immunobiologist and allergy expert at Yale University. It’s that our immune systems get confused.

  

Our immune system is actually two systems. The innate immune system has been around since life on earth consisted of single-celled organisms floating in the seas. It’s what recognizes the chemical signatures of things that might be a threat. The second, adaptive, immune system has been around for about 400 million years. It keeps track of the threats a creature has encountered in the past and can trigger responses, like creating mucus and triggering sneezes to trap and expel the invaders.

Taken together, these systems are meant to protect us. But in people with pollen allergies, they get triggered by something harmless, said Medzhitov: “Seasonal allergies are exaggerated versions of normally protective defenses.”

  

How pollen is able to trigger our defenses is still somewhat mysterious. According to one popular theory called the “hygiene hypothesis,” our immune systems aren’t as well trained as they used to be. “Long ago we all lived outside all the time. Nowadays we’re indoor creatures,” said Paul Turke, an evolutionary biologist and pediatrician based in Michigan.

According to the hygiene hypothesis, early childhood infections and exposure to disease-causing microbes can help train the immune system to better deal with allergens like pollen.

“A cleaner environment sounds positive, but it’s not natural,” Medzhitov said.

But many researchers, including Medzhitov and Turke, think the hygiene hypothesis isn’t the whole story. Exposure to beneficial microbes, for example, is probably just as important, Turke said.

The hygiene hypothesis also doesn’t explain why some people are prone to allergies while others aren’t. Genes certainly play a role but they can’t explain why allergy rates have been increasing in recent decades, Medzhitov said.

Turke thinks the hygiene hypothesis is part of a larger picture: that our modern lifestyle is very different from the conditions in which we evolved. Not only do we live a cleaner and more parasite-free life, we move around frequently, encountering new foods, pollens and other substances that our bodies don’t always know how to deal with.

“Hunter-gatherers who lived in central Europe 20,000 years ago, they didn’t order out for Thai food. They didn’t travel that far,” he said.

In this view, allergies are the price we pay for having radically changed our lifestyle and environment, both outside and inside our bodies. Turke said that doesn’t mean we should ditch antibiotics and go back to a hunter-gatherer lifestyle, but rather be conscious of the fact that our bodies need to be exposed early and responsibly to a wide variety of microbes. 

“It’s a balancing act,” he said.

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