Some people are naturally immune to the symptoms of Alzheimer's even when they have the disease, scientists have found.
Several "super-agers" in their 90s who showed no sign of mental decline or memory loss before their death were all found to have the signature plaques and tangles in their brains which signal Alzheimer's.
Scientists at Northwestern Medicine in the US said they were amazed by the findings and are now looking for genetic, dietary or environmental reasons responsible.
"This is amazing," said lead investigator Professor Changiz Geula. "We never expected it.
"It tells us there are some factors that are protecting their brains and memories against the Alzheimer's pathology of plaques and tangles. Now we have to find out what those are.
"We will look at genetic, dietary and environmental influences that could confer protection for neurons against Alzheimer's pathology."
If scientists can find a protective factor, it could help the normal elderly and those with the Alzheimer's pathology to avoid the devastating symptoms.
The post-mortems showed that despite having dementia the area of the brain responsible for memory formation was still largely intact.
"These findings clearly demonstrate the brains of some elderly are immune to the toxic effects of plaques and tangles," added Geula.
Northwestern scientists studied the brains of eight individuals older than 90 who were selected for superior performance in memory tests compared with their same-age peers who had a normal memory test performance.
Three of those brains qualified pathologically as having Alzheimer's disease, despite superior memory performance of the individuals when they were alive.
When Geula and colleagues examined nerve cells in the hippocampus, the part of the brain that is responsible for memory formation, they found cells in this area were relatively intact in brains of elderly with full Alzheimer's pathology and superior memory performance.
They also examined five brains of Alzheimer's dementia patients with full Alzheimer's pathology.
Those brains showed significant cell death in the hippocampus. A similar pattern was observed in other areas of the brain that control cognitive function.
To count the neurons, they examined a series of tissue sections, which were stained to visualise neurons.
Then, using a microscope, they counted the number of neurons in sections of the hippocampus and the frontal cortex.
When plaques and tangles appear in the frontal cortex, it means Alzheimer's pathology has spread throughout the brain.
Geula's lab is now embarking on a large-scale study to determine the factors, including genetic factors, that help protect the brains of some elderly against Alzheimer pathology.
The findings were presented at the Society for Neuroscience 2016 Annual Conference in San Diego.