A new Japanese study may explain why women live longer than men.
The findings published in the journal Immunity & Ageing suggest one reason is because women's immune systems age more slowly.
The body's immune system wards off disease and infection like cancer, but actually causes disease when not regulated properly.
Japanese researchers found as they age, men are more susceptibility to disease as their immune system weakens at a more rapid rate.
Women, in fact, outlive men by a number of years, and that varies by country.
For example, in the UK, the average life span is 82 for women and 79 years for men.
However, in Japan, where the study originated, women live to 85.5, while men live to only 79 on average.
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In the study, Professor Katsuiku Hirokawa from the Tokyo Medical and Dental University and his team measured the white blood cells and cytokine levels of 356 healthy men and women between the ages of 20-90 years old.
Then, they examined core immune system indicators - T-cell and B-cell levels - and found, as they aged, women had much higher levels that men.
Finally, the researchers discovered that women as they age maintained higher levels of CD4 T-cells, which helps the immune system fight off attacks in the body.