Scientists have found that women's immune system age slower than men's, providing an explanation for why women live longer than men.

It has been proved that women live longer than men and scientists have uncovered one of the possible reasons for this. According to a new study conducted by researchers from Japan, women's immune systems remain younger for a longer time than men's.

The research undertaken at the Tokyo Medical & Dental University looked into the blood samples of 356 healthy men and women aged between 20 and 90. For both genders, it was observed that the number of white blood cells decreased with age.

The number of neutrophils, the most common type of white blood cell in the body was found to decrease with age for both sexes, while the number of lymphocytes (another type of white blood cell present in the vertebrate immune system) was found to increase in women and decrease in men with age. At younger ages, men are known to have a higher number of lymphocytes than women of the same age.

Through the study, researchers found that the rate of decrease of T cells and B cells (groups of white blood cells also called lymphocytes) is higher for men than women. They also found that the declining rate of IL-6 and IL-10 (types of signalling molecules) with age was worse for men than women.

"The process of ageing 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," Professor Katsuiku Hirokawa from the Tokyo Medical & Dental University Open Laboratory explained. "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."