A study may have discovered why breastfeeding might help protect children against allergies such as asthma, scientists have said.
The French research, published in Nature Medicine, shows female mice exposed to allergens can pass them directly to their offspring in milk.
This allows the newborns to become "tolerant" of the substance.
However, in humans, the link between breastfeeding and reduced asthma risk remains unproven, say experts.
There is some research evidence that being breastfed lowers the risk of becoming asthmatic but other studies have failed to find this.
More than 300 million people worldwide have allergic asthma and some scientists believe exposure to allergens, or a lack of exposure, at a very young age may be important in its development.
Asthma happens when the body's own immune system recognises as "foreign" a common and harmless substance found in the environment, such as dust mite faeces.
When this substance is inhaled, the immune reaction can cause inflammation in the airways, narrowing them and making it harder to breathe.
For many sufferers, this can mean a lifetime of drugs, both to damp down the immune reaction and to re-open their constricted airways during an attack.
The researchers, from the INSERM institute in France, used an allergen called ovalbumin - a protein found in egg whites.
They allowed the mothers of newborn mice to breathe in the protein but not their offspring.
Tests confirmed the allergen was then transferred to the baby mice via breast milk and that the baby mice developed an immune system tolerance to it.
This effect happened independently of the mother's own immune system.
The researchers wrote: "This study may pave the way for the design of new strategies to prevent the development of allergic diseases."
Sally Rose, an asthma nurse specialist at Asthma UK, said: 'While some research does suggest that breastfeeding may help reduce the chance of babies developing allergic conditions such as asthma, there are other studies that contradict this.
"Because breastfeeding provides many proven benefits for babies, current advice from the Department of Health, which Asthma UK supports, is that, where possible, babies should be exclusively breastfed for the first six months of life."
Dr Charles McSharry, an immunologist from Glasgow University, said the research did offer a theory as to why breastfeeding might be beneficial in humans.
However, he said comparing the immune reactions of mice and humans was difficult.
"It is far more difficult to induce the kind of immune tolerance they have achieved in mice in humans, which is a key difference," he said.