When a mother is exposed to pollens, moulds and dust, her milk passes these allergens which ward off future allergies, they discovered.

The mechanism's discovery could pave the way to improved formula milk and new methods to prime a baby's immune system so they do not suffer from asthma.

About five million people in Britain, including 1.1 million children, receive treatment for asthma, brought about when an allergen causes the airwaves to narrow, making it hard to breathe. This is known as anaphylactic shock and can be life-threatening.

It has been known for years that breast-fed babies experience fewer and less serious incidences of allergy than formula-fed babies. Admittedly, the results of studies have been conflicting and little has been known about allergens the mothers have been exposed to and which ones they tolerate.

Now the reason why breast feeding protects babies against allergic asthma is reported by a French team in the journal Nature Medicine.

Using experiments on mice, the scientists found that airborne allergens such as pollen, spores and dust are able to pass from mother to child through breast milk, which creates a tolerance to the allergen.

Dr. Valérie Julia, from the Université de Nice-Sophia-Antipolis, Valbonne, and her colleagues investigated whether exposing lactating mice to an airborne allergen - a protein called ovalbumin - affected asthma development in the offspring.

"We found that airborne antigens are efficiently transferred from the mother to the pup through milk," she said.

The ovalbumin transferred from the mother to the pup and trained their young immune system to tolerate the allergen. Studying the effect in detail, they found that a protein called transforming growth factor-beta, TGF beta, which suppresses the immune system, and white blood cells called CD4+ T lymphocytes were responsible for training a young immune system not to over-react to an allergen.

The result is protection from allergic airway disease.

From the February 4, 2008, Prepared Foods e-Flash