It is allergy season once again. If you are one of the 81 million Americans with hay fever, spring is a mixed blessing. Yes, the days are longer, but they are accompanied by itchy eyes, runny noses, and an endless hunt for antihistamines. On days when the pollen count is highest, seasonal allergies are like an assault—from the outside world, but also from our own bodies’ immune systems going into overdrive.

There are growing numbers of allergy sufferers, too. In 1997, around 0.4 percent of US children were reported to have a peanut allergy. By 2008 the figure was 1.4 percent. In the UK, hospital admissions due to severe food allergies tripled between 1998 and 2018. And although rates of asthma—often triggered by allergies—have leveled off in the US, they are continuing to rise globally thanks to increased rates in the developing world. We’re also seeing a rise in unusual allergies, such as alpha-gal syndrome, where some people bitten by lone star ticks develop strong reactions to red meat.

Looking at the rise in allergies, it’s hard to shake the feeling that something is out of kilter. Either it’s the outside world, our bodies, or the complex interaction between the two, but something is going wrong. The question is why—and what can we do about it?

A good place to start is by figuring out what the hell allergies actually are. In her book Allergic: How Our Immune System Reacts to a Changing World, medical anthropologist Theresa MacPhail attempts to do just that. One theory is that allergic reactions evolved as a way for the body to expel carcinogens and toxins—from insect stings to snake bites. Even a few centuries ago, an extreme immune response to a potentially fatal snake bite might have been a useful way for the body to respond, one researcher tells MacPhail.

As the world has changed, our overactive immune systems have started to seem decidedly out-of-step with the threats we face. It doesn’t help that growing seasons for crops are getting longer, exposing people to pollen earlier each spring. At the same time, changing diets and lifestyles are putting our microbiomes out of whack, perhaps making children more likely to become sensitized to food allergens. Stress might also influence our susceptibility to allergies—we know that stress hormones provoke a similar kind of response in mice cells as allergic stressors.

If this is sounding a bit inconclusive, then you’d be right. As MacPhail discovers, it’s hard to pin down exactly what is causing the rise in allergies—doctors don’t even completely agree on what an allergy is or how best to diagnose one. But MacPhail has a good reason to dive into these complexities. In August 1996, her father was cruising down a New Hampshire road on his way to a beach with his girlfriend. A solitary bee flew through the open sedan window and stung him on the side of the neck. Soon afterward, her father died from anaphylactic shock; he was 47. “You are really here today because you want to know why your father died,” one allergy doctor tells MacPhail during an interview.

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