In an orchestra, the conductor doesn't play a single instrument, yet the loudness of the music depends on them. The same goes for the brain's immune cells, called microglia: they don't start the disease, but they can make its blow harder.
Scientists took the brains of 441 people who had donated them to science after death. The average age of these people was 90 years. The brains were scanned with a tomograph to measure the hippocampus, the region that stores memories. Then thin slices were stained to reveal microglia, and a computer counted these cells. It turned out that if the TDP-43 protein accumulates in the brain, then the more microglia there is, the smaller the hippocampus. But for Alzheimer's proteins, there is no such link.
So drugs that calm microglia may help not everyone, but only those who have TDP-43 in their brain. For now, such people are hard to find while alive, but scientists are looking for ways. And then treatment will become not "for everyone" but "for those who need it."
The average age of participants at death was 90.1 years, and the average duration of education was 16 years. These are among the oldest and best-educated cohorts in the world.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.