By Helen Briggs BBC News
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.
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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
Prof Tom Kirkwood University of Newcastle upon Tyne
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."
Life expectancy
Life expectancy in the UK is 79 years for men and 82 for women
In Japan, the gap is wider, with women living to 85.5 on average
Source: World Health Organization
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.