For years, a prevailing theory has been that one of the chief villains in Alzheimer’s disease has no real function other than as a waste product that the brain never properly disposed of. The material, a protein called beta amyloid, or A-beta, piles up into tough plaques that destroy signals between nerves. When that happens, people lose their memory, their personality changes and they stop recognizing friends and family. But now researchers at Harvard suggest that the protein has a real and unexpected function — it may be part of the brain’s normal defenses against invading bacteria and other microbes....