Interestingly, performing the reverse, in which a young mouse was injected with blood (or, more accurately, plasma, which is the parts of blood without blood cells), resulted in young mice with distinctly elderly attributes--increased inflammation, a reduction in the production of new neurons, that kind of thing. The researchers used plasma because blood cells are actually too large to travel through the blood-brain barrier into the brain. But certain chemokines, small proteins secreted by cells, are indeed small enough to pass through, and the team actually isolated several that could be causing this effect.