Dear Science

Dear Science,

I moved to Seattle from the East Coast six years ago and immediately found relief from the seasonal allergies I suffered in the spring and summer. During a recent trip back East, I found the breathing easy—no sneezing, wheezing, or itchy eyes. Now, however, I am back in Seattle and miserable from raging allergies. What gives? Why have my allergies now adjusted to where I live?

Sneezy

To understand allergies, we must get to know pathogenic worms. Let’s meet an exemplar nematode family, the schistosomes—200 million of your human compatriots already have. These delightful pathogens enjoy life in both humans and freshwater snails. Infected snails release one form of these worms (called cercariae) into the water. If a human being swims in this water—or, in endemic areas where public bathrooms are rarer than in Manhattan, bathes and bathrooms in the freshwater—the cercariae can shove their way through intact human skin, depositing the head of the worm into the blood. Via the blood, the worms find the liver, deposit themselves, and mature into adults. The adults leave the liver, in male and female pairs, and travel to the bowel blood vessels or the bladder, where they mate and release eggs into the feces or urine. These eggs, if they make their way into fresh water, hatch into a form called miracidia, capable of infecting snails. The cycle continues.

Until very recently, dealing with worms like these was a primary task of your immune system. Handling this sort of invasion—by a large, hostile, multicellular organism—is quite different than dealing with the infections of bacteria and viruses that we’re still beset with. Our immune systems have two entirely different playbooks. For bacteria and viruses, it’s all about gobbling up the tiny pathogens with immune cells and killing off our own cells that are infected. For worms, our immune systems attempt to wall off the beasties within us and slick up our barriers to deny the worms purchase as they try to get in or hold on. Think of allergy symptoms: runny nose; diarrhea; swollen, itchy, and watery eyes; and endless sneezing. These are all great ways to annoy worms seeking purchase in us.

Allergies happen when our worm-defense system becomes ramped up against something innocuous—an allergen. Why this happens is probably a combination of your genes, pollution (irritating your body in ways that are misinterpreted as a worm attack, perhaps), and previous real infections that looked like this allergen. When you moved to Seattle, the potential allergens completely changed; it took a while for your immune system to get confused again and learn a whole new set of false worms. Move again, and the cycle will continue. For now, Science suggests an antihistamine and a neti pot.

Wheezily Yours,

Science

Send your science questions to dearscience@thestranger.com.