The latest research into childhood allergies suggests that having a pet in the home reduces the risk of children being sensitive to allergens.
The scientists from the National Research Centre for Environmental Health in Munich say they have found that having a pet when a child is growing up trains the immune system to be less sensitive to potential triggers for allergies such as asthma, eczema and hay fever.
Childhood allergies have risen markedly in the last decade and the range of possible reasons offered by experts is varied and often confusing and the issue is a controversial one.
Previous generations appeared less affected by allergies possibly because they were exposed to more dirt and the micro-organisms within it, which helped their immune systems develop resistance.
The German team arrived at their conclusion after a six-year study monitoring the health of 9,000 children, and they say having a dog provides enough of the right kind of dirt for babies and the presence of a dog in the home during a child's infancy is linked with a significantly low level of sensitisation to pollens and inhaled allergens.
Lead researcher Professor Joachim Heinrich says their research supports the theory that modern life has simply become too clean and babies' immune systems are not exposed to enough germs to develop normally.
Professor Heinrich says the same protective effect was not seen in children who had frequent contact with dogs but none at home.
Previous research has also suggested that exposure to pets may have a protective effect against allergies but this latest research differs in that it was not based on retrospective questioning of subjects about their exposure, but the children were followed from birth to the age of six, producing more reliable results.
According to Heinrich and his team the blood of children raised in households with dogs contained fewer markers for allergy, such as antibodies to pollen, house dust mites, cat and dog dander, and mould spores.
The researchers are now planning to run more tests when the children reach the age of ten in order to establish whether the protection stops them developing allergies such as hay fever.
In an analysis of more than a dozen studies involving almost 20,000 children, other research has revealed that children who mixed with other youngsters were 30 per cent less likely to develop acute lymphoblastic leukaemia, the most common form of child leukaemia.
The scientists suggest that exposure to common infections from the early years of life helps ensure that the immune system works properly and recent evidence suggests that early exposure to cats, dogs, and to farm animals is neutral or even protective against all sorts of allergens.
Other research similar to Professor Heinrich's have also suggested that early exposure to cats increases the risk, others that it diminishes it and one study found that asthma symptoms were reduced in homes that owned a dog.
The study is published in the current issue of the European Respiratory Journal.