The new study backs up the theory that modern children are too ‘clean’ for their own good as the immune system can over-react if it doesn’t have enough viruses to fight

Catching viruses such as flu when you’re younger could protect your from developing allergies later in life, a new study has found.

It adds to evidence that a growing number of children in developed countries are suffering from allergies because they are not exposed enough to bacteria and viruses.

In the latest research, scientists from Harvard Medical School found baby mice infected with the influenza A virus grew into adults that were protected against induced symptoms of the condition.

However, adult animals catching flu remained susceptible to asthma.

Scientists traced the effect to a sub-group of immune system cells called natural killer T-cells (NKT cells).

Some NKT cells appear to keep the immune system under control and prevent the extreme reactions that lead to allergies such as asthma. In the baby mice with flu, but not the adults, numbers of these cells were found to increase.

The research also showed that NKT cell protection against asthma could be induced by exposing baby suckling mice to a molecule from the stomach bug Helicobacter pylori.

The findings, published in the Journal of Clinical Investigation, provide strong evidence in support of the ‘hygiene hypothesis’ – the theory that we are too clean for our own good.

Early infections may ‘prime’ the immune system to prevent it over-reacting to allergy triggers later in life, according to the theory.

The scientists, led by Dr Dale Umetsu, wrote: ‘Our results suggest that infection with certain micro-organisms can prevent the subsequent development of asthma and allergy by expanding the relative proportion of a specific subset of NKT cells, thus providing an immunological mechanism for the hygiene hypothesis.’

Treating children with drugs that activate protective NKT cells may block the development of asthma, the researchers concluded.