Human blood enables vampires to remain physically attractive and mentally sharp according to ancient lore. But the anti-aging properties of blood may be more than a legend. Researchers at the biomedical startup Alkahest in California are actually running a small clinical trial that involves injecting human blood plasma from young people into dementia patients. The hope is that factors in the blood of young people will repair and rejuvenate ailing brains. It worked in mice, so maybe it will work in people, writes Ronald Bailey.