For decades, most researchers have agreed that these plaques and tangles are the key malefactors of dementia, and that if you could clear them from the brains of patients, you would halt or reverse illness. Researchers have been especially focused on finding a drug that could erase amyloid plaques, and we now have dozens of compounds that do that in mice.

But this approach has led to failure in humans. Even when drugs can clear the plaques in patients’ brains, the disease continues to wreak damage.

Now some scientists believe that the focus on amyloid plaques might have been a mistake. Instead of looking at what goes wrong, they’re trying to understand what goes right.

Changiz Geula, a professor of neuroscience at Northwestern University, has been studying brain tissue collected from people who died at age 90 or older. He found that some people who die with sharp minds have brains that are clogged with the gunk associated with Alzheimer’s pathology. That means it’s possible to have an “Alzheimer’s brain” but no dementia. Dr. Geula believes that in cases like this, some actor in the brain — call it the opposite of Alzheimer’s — is protecting neurons from damage. We still don’t know what it is.

One candidate might be the astrocytes, cells that support the neurons and synapses, keeping them healthy even in the presence of plaques and tangles. In a 2017 paper in Nature, Stanford University researchers described how these usually peaceable cells can flip into a “killer mode,” becoming assassins that spew out toxins and destroy the very cells they once nursed.

According to Shane Liddelow, one of the authors of the paper, this Jekyll-and-Hyde personality of the astrocytes likely developed thousands of years ago to fend off the infections that invaded the brains of our ancestors. At the first sign of trouble, the astrocytes go on the attack, destroying everything in their path — including sometimes healthy brain tissue. Neurons can become “innocent bystanders in this protective killing effort,” Dr. Liddelow explained.

Nowadays, since most of us live in more sterile environments, this army in our brain is no longer busy fighting pathogens, and so it responds instead — often far too vigorously — to the amyloid plaques and tangles that are a part of normal aging.

“Ten years ago, very few scientists were looking at whether the immune system was related to Alzheimer’s, but that question has just exploded,” Dr. Liddelow said. “At every scientific meeting I’m at, everyone’s talking about this question: Why are some people with lots of amyloid plaques — the people who, according to our models, should get Alzheimer’s — protected from this runaway immune response? I think the answer will come from looking at immune cells of humans around the world living in different environments.”