Transplanting blood cells from young mice into old mice prevented cognitive decline by halting the activation of a type of immune cell in the brain called microglia. This allowed more synapses in the brain to be preserved during the aging process.

There have been several studies that have shown similar results and multiple mechanisms seem to be at play. There are a couple of clinical studies underway that are looking at transplanting blood from young individuals into older individuals with mild cognitive dementia to see how this affects cognitive function.