The life expectancy gap between men and women is a rather attested fact, and while in the past a laborious, physically tense lifestyle for men was used to serve as an explanation, in our day and age of gender equality this doesn’t quite cut it anymore. Researchers in Japan might have stumbled across a clue that explains why women life longer then men, after they found that at old age women’s immune system is stronger than men’s.

In the United States, based on a 2005 census, the life expectancy gap between men and women is 5.3 years, with women expected to live on average up to 80.1 years. In Japan, a country which holds the record for life expectancy among the world’s populations, the gap is even wider with women being expected to live on average six years longer than men.

Many theories have been proposed in attempt to explain this gap. In fact, it turns out the females of most species live longer than males, which might mean that the life expectancy gap might be deeply rooted in our biology. A theory says women live longer men because they’re less ‘disposable’. Bear with me. The idea is that men’s reproductive role is less dependent on health than that of women, and as such the biological repairing mechanisms (DNA and cell division to replace old and dead cells) might be more refined in women. Other ideas state that male testosterone is actually harmful in the long run and may reduce life expectancy, although tangible evidence is insufficient to back this up.

Scientists at Tokyo Medical & Dental University have a different explanation, and their research has provided evidence to that shows women’s immune system at old age is stronger than in men. The findings were made after the researchers examined the blood of 356 men and women aged between 20 and 90 years old, looking to study key immune system signals – the levels of white blood cells and cytokines, which help to carry messages in the immune system.

As expected, in both sexes the number of white blood cells decreased with age, but two key elements – known as the T-cell and B-cell lymphocytes, declined faster in men. They 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. Moreover,  two types of cytokines that help to keep the immune system under control and prevent inflammation from damaging surrounding tissue showed a decline only in men.

“It is well known that ageing is associated with a decline in the normal function of the immune system, leading to increased susceptibility to various diseases and shortened longevity,” said Professor Katsuiku Hirokawa.

“However, specific dysfunctions in the immune system directly responsible for this have yet to be identified.

“Among the important factors, T cells are central to the immune response, and their function is significantly altered with increasing age.”

Besides trying to explain the life expectancy gap, the findings also could serve a better indicator for calculating biological age, based on the state of one’s 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,” Hirokawa said.

The findings were reported in the journal  Immunity and Ageing.