A woman's immunity stays younger longer and this may contribute to women living longer than men, researchers in Japan say. Katsuiku Hirokawa of the Tokyo Medical & Dental University Open Laboratory said the research team looked at the blood of healthy volunteers in Japan, ages 20-90 of both sexes. They found the total number of white blood cells per person decreased with age. The number of neutrophils -- the most abundant type of white blood cell -- decreased for both sexes, while the lymphocytes, a type of white blood cell in the vertebrate immune system, decreased in men and increased in women. Younger men generally have higher levels of lymphocytes than women of a similar age, so as aging occurs, the number of lymphocytes becomes comparable, Hirokawa said. There was also a age-dependent decrease in red blood cells for men but not women, the study said. "The process of aging is different for men and women for many reasons. Women have more estrogen than men which seems to protect them from cardiovascular disease until menopause," Hirokawa 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." The findings were published in the journal Immunity & Ageing.