Pets do an awful lot for kids: they teach them about unconditional love, responsibility, death and, of course, pooper scoopers. But does a dog or cat also keep a child from developing allergies? Despite decades of research, the short answer is still a frustrating "maybe."

The idea that pets might provide an immune benefit stems from a controversial theory born in 1989 called the hygiene hypothesis. It postulates that the sharp rise in allergic diseases this century can be explained, at least in part, by our higher cleanliness standards. Microorganisms like bacteria and parasites are thought to "prime" our immune systems to fight the important fights—dangerous infections—while smartly ignoring the frivolous ones. Allergies, then, occur because naive immune systems unnecessarily attack harmless environmental triggers, such as pet dander. Today, most researchers believe that only certain microorganisms, such as parasitic worms and lactobacilli, play a role in prepping the immune system. The question is whether pets provide some of them.

Studies designed to address this question, however, have been anything but conclusive. "Welcome to a complex field," says Thomas Platts-Mills, head of the allergy and clinical immunology division at the University of Virginia. For one thing, because cat and dog allergens like dander and saliva travel easily on clothing and in the air, most children are exposed to them—so it's not a black-and-white issue of exposure versus nonexposure. "The question is really, what happens at much higher exposure, that is, when there is a cat in the home?" says Matthew Perzanowski, an environmental health scientist at Columbia University's Mailman School of Public Health.

According to Perzanowski, pet ownership appears to be associated with a decreased risk of developing allergies in some, but not all, communities. For example, in countries with lots of cats, like the suburban U.S. and Australia, a pet cat appears to provide a protective effect. But in countries with few cats, he says, ownership actually increases the risk for allergic sensitization, the immune reaction that often precedes allergy symptoms. And don't even try to understand countries with moderate cat ownership—studies have been inconclusive. As for why these differences exist, no one really knows.

There's stronger evidence for protection from dogs, says Augusto Litonjua, an associate physician at Brigham's and Women's Hospital in Boston, but he admits that there could be a number of reasons for this. For instance, he says, how can you distinguish between the direct effect of owning a dog and the lifestyle choices that accompany it, such as going outside more, being more physically active, and absorbing more sun and vitamin D? It may not be the dog per se that's having an effect, he says.

But there is some molecular evidence for dog-specific protection. Dogs are known to carry lots of bacterial compounds called endotoxins in their fur, Litonjua says. His lab's studies have shown that when cells from children who have grown up in homes with higher endotoxin levels are directly exposed to allergens in the laboratory, they release fewer cytokines, chemicals associated with allergic responses.

Endotoxin exposure might also explain the well-supported finding that children raised on farms experience fewer allergy symptoms than other kids. Farms are chock-full of microorganisms and animals, and these might confer a protective effect; but again, farm children lead very different lives than kids raised elsewhere, so it's difficult to tease out exactly what is protecting them, Litonjua says.

Indeed, some scientists urge caution when it comes to interpreting the majority of pet allergy studies out there. In many, researchers questioned groups of people about their pet ownership patterns and compared this with their allergy profiles.

Such studies don't prove causation, says Carl-Gustaf Bornehag, a public health scientist at Karlstad University in Sweden. Often, people who are allergic to pets—or, because allergies are at least partially hereditary, people who are at risk of becoming allergic because someone in their family is—simply aren't going to own them, he says.

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