This light micrograph of a part of a brain affected by Alzheimer’s disease shows an accumulation of darkened plaques, which have molecules called amyloid-beta at their core. Once dismissed as all bad, amyloid-beta might actually be a useful part of the immune system, some scientists now suspect — until the brain starts making too much.

This light micrograph of a part of a brain affected by Alzheimer’s disease shows an accumulation of darkened plaques, which have molecules called amyloid-beta at their core. Once dismissed as all bad, amyloid-beta might actually be a useful part of the immune system, some scientists now suspect — until the brain starts making too muc