Having a dog in the house reduces the risk that young children will develop allergies, German researchers said on Tuesday.

The finding, based on a six-year study of 9,000 children, lends weight to the theory that growing up with a pet trains the immune system to be less sensitive to potential triggers for allergies like asthma, eczema and hay fever.

Just why this should be is unclear but scientists believe youngsters may get beneficial early exposure to germs carried into the house on the animal’s fur, which helps their immune systems develop.

“Our results show clearly that the presence of a dog in the home during subjects’ infancy is associated with a significantly low level of sensitization to pollens and inhaled allergens,” said Joachim Heinrich of the National Research Centre for Environmental Health in Munich.

The same protective effect was not seen in children who had frequent contact with dogs but did not have one at home.

Previous studies have suggested that exposure to pets may have a protective effect against allergies but many of these studies were based on retrospective questioning of subjects about their exposure.

Heinrich’s study, by contrast, was designed before the data was collected. Experts consider such prospective studies make for more reliable results.

Parents answered detailed questionnaires about possible allergic symptoms in their children, from birth to the age of 6, and blood samples were also taken from a third of the group to test for antibodies to common allergens.

The group’s findings were published in the European Respiratory Journal.