Pets: Allergy Friends or Foes?

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Studies suggest household animals, siblings can reduce allergy risk

For years, my allergists insisted I find new homes for the pets that make me sniffle and sneeze. Those little dander machines are just making your allergies worse, they intoned. I refused, being rather attached to the little fur balls.

Right now I'm feeling pretty darned smug. Recent studies suggest that household pets may actually prevent allergies from developing — in children at least.

One study by Danish researchers, published in April in the British Medical Journal, scrutinized more than 24,000 mother-and-child pairs. It found that kids who grew up in a home filled with furry pets were less likely to develop skin rashes that commonly occur in those prone to allergies. Having a household full of siblings also seemed to be protective, researchers reported.

Another study, published in February in the Journal of Allergy and Clinical Immunology, showed that kids who grew up with a dog in the house were less likely than others to develop the same kind of skin rashes. These children also had higher levels of a substance called interleukin-10 (IL-10), an immune system hormone that quiets inflammation.

Out on the farm
As it turns out, there's a whole school of thought now suggesting that, as a society, we're becoming increasingly more allergic because we keep our houses — and our children — too tidy.

The theory evolves from research that found that children who grew up on farms tended to be less allergic than those who grew up in the city (or even the country, but not on a farm), says Dr. David Resnick, an associate clinical professor of pediatrics in the division of allergy at the New York Presbyterian Hospital.

What you have to understand is that there are two types of immune responses, Resnick says. One, which is organized by T-helper-1 cells, is a normal response, generally directed against bacteria and viruses.

The other type of response, which is orchestrated by T-helper-2 cells, leads to the production of a substance called IgE, the antibody involved in allergic reactions.

Some scientists think that the T-helper 2 system was originally needed to fend off infections involving parasites. That's because IgE levels go up when a person is infected with a parasite, such as giardia, Resnick says.

Some scientists suspect that when there aren't enough bacteria around to challenge the T-helper-1 system, the T-helper-2 system becomes overdeveloped, says Dr. Andre Nel, a professor of medicine at the UCLA School of Medicine.

"When there's a pet in the house, the house tends to be less hygienic," Nel explains. That's because animals shed not only fur, but also endotoxins, substances found in the coating of bacteria.

So, it's possible that when the body has bacteria to react to, it doesn't mount an allergic response, Resnick says.

The tolerance theory
It's a nice theory, says Nel. But not everyone believes it.

Some researchers believe that the explanation may simply be that children who grow up with a pet adapt to the pet's presence in a process called "tolerance."

The process may be similar to what happens when doctors treat allergies with injections of increasingly high doses of the allergy-causing proteins, Nel says.

Well, I personally happen to like that explanation better. It was with great pleasure that I pointed out to my latest allergist that I used to test positive to allergy for cats. My last test showed no such sensitivity.

One problem with the tolerance theory is that it doesn't seem to work with pollens, Nel says. Even in Southern California where grass pollens seem to be in the air almost continuously, these allergens will still spark itchy eyes and runny noses.

Pet peeves
Whatever theory you subscribe to, researchers say you shouldn't go out and buy a pet just to stave off pet-related sniffles and sneezes in your children.

"Every allergist has seen kids who grew up with pets and had no problem with allergies," Resnick says. "But then they go away to college and when they come home they are suddenly terribly allergic to the pet."

Further, Resnick says, "studies show that in the case where the mother has a history of asthma and the child grows up in a home with a cat from day one, there is a higher incidence of allergies and asthma."

Sounds like I was just lucky that it was my dad and not my mom who had all the allergies. Good news for all the furry critters frolicking around my house.