This still doesn't explain why some people are more allergy-prone than others. "Allergens are everywhere," says Erika von Mutius, an allergy specialist at Munich University Children's Hospital in Germany. "So if this is a defense, why isn't everybody allergic?" According to Medzhitov, allergies may be more common in people with defects in other defensive tactics. For instance, 42 percent of people who have a mutation in a structural skin protein called filaggrin commonly experience allergic skin reactions. "If you don't have optimal physical barriers, you rely on a greater degree on allergic defenses," he says.
And what about the growing body of research suggesting that childhood environment shapes allergy risk? A 2011 study published in The New England Journal of Medicine reported that children who grow up on farms, where they are exposed to many microorganisms, are less likely than other kids to develop asthma and allergies. This idea, known as the hygiene hypothesis, suggests that individuals who encounter a multitude of bacteria and viruses early in life invest more immune resources into type 1 responses at the cost of type 2 reactions, including allergies. Medzhitov maintains that this theory can co-exist with his own. "It's a different aspect of disease susceptibility that has to do with early programming," he says.
Ultimately, Medzhitov's theory raises more questions than it answers, but many agree that the basic tenets are plausible. "It stimulates us as scientists to draw up some new hypotheses," says Kari Nadeau, an immunologist at the Stanford School of Medicine. "The hypotheses need to be tested and might not necessarily be confirmed, but at least this paper drives us to understand allergies better."