Demographic statistics in the developed world have long told us that women live longer than men, but scientists have often been left scratching their heads when trying to figure out why.
Researchers discovered that levels of white blood cells and cytokines, which help to carry messages in the immune system and are responsible for fighting off bacterial infections, decrease more quickly in men than in women.
The findings were published in the open access journal Immunity & Ageing.
"The process of aging is different for men and women for many reasons," explains lead researcher Katsuiku Hirokawa from the Tokyo Medical and Dental University. "Women have more estrogen than men which seems to protect them from cardiovascular disease until menopause. Sex hormones also affect the immune system, especially certain types of lymphocytes."
Hirokawa and his team examined the blood of 356 Japanese volunteers, who ranged in age from 20 to 90 years old. While the total number of white blood cells decreased with age in everyone, the rate of decline for two key types of cells — T-cell and B-cell lymphocytes — was slower in the female subjects.
The researchers also found that another type of cell that tackles viruses and tumours increased with age, with women having a higher rate of increase than men.
Red blood cells also decreased in men over time, but not in women.
Hirokawa believes his research will eventually help experts assess an individual's "biological age" based on the immune system.
"Because people age at different rates, a person’s immunological parameters could be used to provide an indication of their true biological age," he explains.
Professor Tom Kirkwood of the Institute of Ageing and Health at the University of Newcastle upon Tyne, tells BBC News that the findings, while valuable, are not 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 speculates.
According to the most recent Stats Canada reports on life expectancy at birth, Canadian women are expected to live, on average, 83 years — four years longer than Canadian men.