THE vampire jokes write themselves. In the past few years a steady trickle of scientific papers has suggested something straight out of an airport horror novel: that the blood of young animals, infused into the old, has rejuvenating effects. Scientists are excited enough that at least two clinical trials are currently running in humans. But is it true? And if it is, how does it work?

The answer to the first question seems to be a qualified yes, at least in animals. The rejuvenating effects are seen when lab mice are joined together in a rather gruesome procedure called parabiosis. That involves making cuts in the skin of two animals, then suturing them together at the site of the wound. As the cuts heal, the pair’s blood vessels will grow together and merge. The result is two animals that share a circulatory system, with both hearts pumping both sets of blood around both bodies. Doing this with an old mouse and a young mouse has some spectacular effects. As with humans, old mice have a harder time healing from injuries. But link an old mouse to a young one and it becomes able to repair muscle injuries nearly as well as its younger counterpart. Similar benefits are seen in liver cells and the nervous system. And it works in reverse, too: old blood can have a decrepifying effect on the young.

Exactly how this all works is much less clear. The best guess is that some combination of hormones, signalling factors and the like in the young blood affect the behaviour of stem cells in the old animal. Like everything else, stem cells—which are vital for healing wounds and for general maintenance—begin to fail with age. But that process seems to be reversible, with young blood restoring the cells’ ability to proliferate and mend broken tissue. Another theory is that the old animal benefits from access to the organs (kidneys, liver and so on) of its young companion. It may be that both are true: experiments in which animals are given quick transfusions, rather than being stitched together for weeks, still show benefits, though not as many as with full-on parabiosis.

That uncertainty has not stopped people jumping into trials with humans. One company, called Alkahest, has recruited 18 people with Alzheimer's disease. It plans to give them regular blood transfusions from young donors. The trial is primarily designed to prove that the treatment is safe. But because blood transfusions are already routine, Alkahest hopes that will be easy, and plans to look for mental benefits, too. Another company, Ambrosia, has raised eyebrows by charging people $8,000 to take part in its clinical trial, which will see people over 35 receiving blood from under-25s. It is far from clear whether any of this will work (anti-ageing research is dogged by cycles of hype and disappointment). And if it does, there is already a perennial shortage of donated blood, and that is needed for surgery and medical emergencies more than for speculative anti-ageing therapies. The best-case scenario is that blood compounds do indeed turn out to be responsible for the salutary effects; scientists can identify them; and biochemists can work out a way to mass-produce them as drugs. Even then, the result would not necessarily be a life-extension potion. The hope instead is to extend “healthspan”, keeping elderly people hale and hearty for longer. Which would itself be quite something.