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Women's immune systems stay younger for longer, a new study reveals.

Scientists say that woman's more youthful immune systems may partly explain why women generally live longer than men.

The latest study published in the journal Immunity and Ageing analyzed the blood of 356 healthy volunteers between the aged 20 to 90 years old.

Researchers found that the total number of white blood cells per person decreased with age in both sexes.  However, while the number of neutrophils decreased for both men and women, the number of lymphocytes decreased in men and increased in women. 

Researchers explain that younger men generally have higher levels of lymphocytes than similarly aged women, but as time goes by the number of lymphocytes in both sexes become more similar.

Further analysis shows that the drop in T cells and B cells was slower for women, and while both CD4+ T cells and NK cells increased with age in both sexes, the rate of increase was higher in women than men. The study also found that an age-related decline in IL-6 and IL-10 was worse in men, and that there was an age-dependent decrease in red blood cells for men but not women.

Researchers explain that the difference in the decline of immune systems between aging men and women is one of many processes that change as people age.

"The process of aging is different for men and women for many reasons. Women have more oestrogen than men, which seems to protect them from cardiovascular disease until menopause," researcher Prof Katsuiku Hirokawa from the Tokyo Medical & Dental University Open Laboratory said in a statement.

"Sex hormones also affect the immune system, especially certain types of lymphocytes. Because people age at different rates a person's immunological parameters could be used to provide an indication of their true biological age," he added.

"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," researchers wrote in the study.