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 defences against invading bacteria and other microbes.
Other Alzheimer's researchers say the findings, reported in the current issue of the journal PLoS One, are intriguing, though it is not clear whether they will lead to new ways to preventing or treating the disease.
The new hypothesis got its start late on Friday evening in the summer of 2007 in a laboratory at Harvard Medical School. The lead researcher, Rudolph E. Tanzi, a neurology professor who is also director of the genetics and aging unit at Massachusetts General Hospital, said he had been looking at a list of genes that seemed to be associated with Alzheimer's disease.