It’s fairly common knowledge that women tend to, as a general rule, outlive men. Now research completed in Japan is hinting at why. Women tend to have stronger immune systems for longer, which makes them more able to fight off and recover from disease until a more advanced age. This may be the key to their greater longevity. The research found that as women aged, multiple important kinds of white blood cells, those little warriors responsible for defending against infectious diseases, decreased at a lower rate in women than in men. Additionally, another type of immune cell that fights viruses and tumors increases more quickly in women than in men. The combination results in a stronger immune system as women grow older than men.