KUSA - On the average women outlive men. That has been going on for as long as history remembers. But the big question is: why?

One clue came about recently when scientists found that women's and men's immune system functions declined at different rates as we age. And a new Japanese study uses this difference in immune system functions between the genders to add more evidence to our immune system affecting our survival rate.

The scientists involved in this study suggest women's immune systems may age slower than men's. This more gradual decline could explain why women tend to have a longer lifespan.

When researchers examined blood from adults between 20 and 90 years old, they found white blood cells decreased in both sexes with age. But the decline in disease fighting T-cells and B-cells was slower for women than men. That means that women's immune systems stayed more robust as they get older.

Researchers publishing in the journal "Immunity and Aging" set out to exam these differences. They studied healthy Japanese men and women. Their findings confirmed early studies and found that there are age related changes in our immune system and these changes differ between the genders.

Particularly, they found that the rate of decline of functions related to the immune system was greater in men than in women and as we age these differences continue. In other words, men's immune systems weaken quicker than women's do. But they go further in their conclusion and feel that these age-related differences in immune system functions also account s for one reason women tend to outlive men.

According to the Centers for Disease Control and Prevention, the average overall life expectancy in the US is 78.7 years old. But it does depend on whether you are male or female. For males life expectancy is 76.3 years and for females it's 81.1.

There are numerous reasons for this 4.8 year difference, including the fact that females are more likely to seek help for medical issues that may arise and they also tend to take better care of their overall health during their lifetime. But not it now looks like even our own immune systems comes into play as well.

And since men's faster immune function decline seems to be contributing to their lower life expectancy doing what they can to stay healthy can go a long way towards increasing a man's time on earth. That means eating better, exercising more and getting rid of bad health habits like smoking.