On November 9, 1989, an unusual opportunity came along: The Berlin Wall fell. For the first time since the 1940s, West Germans could conduct research in the East. Von Mutius, of Ludwig-Maximilians University Munich, seized the opportunity, expanding her study to include Leipzig, a city of 520,000 deep in East Germany.
The countryside around Leipzig was home to polluting chemical plants and was pocked with open-pit coal mines; many residents heated their apartments with coal-burning ovens. It was a perfect experiment: Two groups of children with similar genetic backgrounds, divided by the Iron Curtain into dramatically different environments. If air pollution caused asthma, Leipzig’s kids should be off the charts.
Working with local doctors, von Mutius studied hundreds of East German schoolchildren. “The results were a complete surprise,” von Mutius says. “In fact, at first we thought we should re-enter the data.” Young Leipzigers had slightly lower rates of asthma than their Bavarian counterparts—and dramatically less hay fever, a pollen allergy.
Puzzling over her results, von Mutius came across a paper by David Strachan, a British physician who had examined the medical records of 17,000 British children for clues to what caused allergies later in life. Strachan found that kids with a lot of older brothers and sisters had lower rates of hay fever and eczema, probably because the siblings brought home colds, flus and other germs.
After learning of Strachan’s study, von Mutius wondered whether air pollution might somehow protect East Germans from respiratory allergies.
Soon, studies from around the world showed similarly surprising results. But it was germ-laden dirt that seemed to matter, not air pollution. The children of full-time farmers in rural Switzerland and Bavaria, for example, had far fewer allergies than their non-farming peers. And a study following more than 1,000 babies in Arizona showed that, unless parents also had asthma, living in houses with dogs reduced the chances of wheezing and allergies later in life. Researchers proposed that the more microbial agents that children are exposed to early in life, the less likely they are to develop allergies and autoimmune diseases later on. Studies also showed that baby mice kept in sterile environments were more likely to face autoimmune disease, seeming to back what came to be called the “hygiene hypothesis.”
“It was so unexpected,” says von Mutius, who now believes air pollution was a red herring. Instead, East German children may have benefited from time spent in daycare.
Think about it this way: At birth, our immune cells make up an aggressive army with no sense of who its enemies are. But the more bad guys the immune system is exposed to during life’s early years, the more discerning it gets. “The immune system is programmed within the first two years of life,” says Knip. “With less early infection, the immune system has too little to do, so it starts looking for other targets.”
Sometimes the immune system overreacts to things it should simply ignore, like cat dander, eggs, peanuts or pollen. Those are allergies. And sometimes the immune system turns on the body itself, attacking the cells we need to produce insulin (Type 1 diabetes) or hair follicles (alopecia) or even targeting the central nervous system (multiple sclerosis). Those are autoimmune disorders.
Both appear to be mostly modern phenomena. A century ago, more people lived on farms or in the countryside. Antibiotics hadn’t been invented yet. Families were larger, and children spent more time outside. Water came straight from wells, lakes and rivers. Kids running barefoot picked up parasites like hookworms. All these circumstances gave young immune systems a workout, keeping allergy and autoimmune diseases at bay.