The key to women living longer may be a stronger immune system.

It is well known that women tend to outlive men.

Perhaps one of the most famous was French woman, Jeanne Louise Calment, who died in 1997 at the age of 122.

But research reported in the New Scientist suggests women may live longer because they produce more immune cells - not because men take more risks as has previously been thought.

Scientists from Imperial College School of Medicine in London studied the thymus gland.

The thymus is located in the lower neck, above and in front of the heart.

It produces T cells, white blood cells which fight off infections, but wastes away after people reach sexual maturity.

T cell decline

Researchers led by Richard Aspinall and Jeffrey Pido-Lopez examined the number of healthy T cells produced in 46 healthy men and women aged between 20 and 62.

They found that T cell production declined in both sexes as they aged.

But women still had higher levels of healthy T cells than men of the same age.

To check if women were therefore better protected against infections, the team looked at deaths from pneumonia and influenza in Britain between 1993 and 1998.

They discovered more men than women died of these diseases, in line with the difference in thymus activity.

Dr Aspinall said: "There are a host of factors that could contribute to susceptibility to infection, of which thymic output might be one.

He admitted that risk taking could play a part, but added factors such as thymic output could contribute to the difference in longevity between the sexes.

Dr Pido-Lopez added: "During old age, when T cells are not as effective, a higher replacement of old T cells with new ones would be a bonus."