Women live longer than men partly because their immune systems age more slowly, a study suggests.
As the body's defences weaken over time, men's increased susceptibility to disease shortens their lifespans, say Japanese scientists.
Tests of immune function could give an indication of true biological age, they report in Immunity & Ageing journal.
The immune system protects the body from infection and cancer, but causes disease when not properly regulated.
The Japanese study set out to investigate the controversial question of whether age-related changes in the immune system could be responsible for the difference in average life expectancy between men and women.
Prof Katsuiku Hirokawa of the Tokyo Medical and Dental University and colleagues analysed blood samples from 356 healthy men and women aged between 20 and 90.
They measured levels of white blood cells and molecules called cytokines which interact with cells of the immune system to regulate the body's response to disease.
In both sexes, the number of white blood cells per person declined with age as expected from previous studies.
However, closer examination revealed differences between men and women in two key components of the immune system - T-cells, which protect the body from infection, and B-cells, which secrete antibodies.
The rate of decline of most T-cell and B-cell lymphocytes was faster in men, while men also showed a more rapid age-related decline in two cytokines.
Two specific types of immune system cell that attack invaders - CD4 T-cells and natural killer cells - increased in number with age, with a higher rate of increase in women than in men.
The researchers believe a person's immunological parameters could provide an indication of their true biological age.
"Age-related changes in various immunological parameters differ between men and women," Prof Hirokawa and colleagues report in the online journal Immunity & Ageing.
"Our findings indicate that the slower rate of decline in these immunological parameters in women than that in men is consistent with the fact that women live longer than do men."
Commenting on the study, Prof Tom Kirkwood of the Institute of Ageing and Health at the University of Newcastle upon Tyne, said the findings, while valuable, were not fundamentally surprising.
"It's likely that the slower ageing in the immune system of women reflects a generally slower rate of intrinsic ageing, rather than that the immune system itself is setting the pace," he told BBC News.
Dr Donald Palmer, senior lecturer in immunology at the Royal Veterinary College, said studies in mice had shown similar results.