One of the key factors that makes the brain age has been uncovered by scientists in the US. Young mice infused with the blood of older animals developed a drop in their cognitive abilities. This is down to a chemical made by the immune system called beta2-microglobulin. Scientists don't know why it has this effect but by removing it, they could reverse cognitive decline and even promote the growth of regenerative stem cells. Chris Smith spoke to Saul Villeda from the University of California to find out more....