In a defense mechanism gone wrong, our brain's own helper cells start to kill off neurons — the brain cells responsible for thinking — when they show signs of Alzheimer's, new research suggests.

Alzheimer's is the most common form of dementia and the sixth-leading cause of death in the U.S., where 5.4 million Americans have Alzheimer's.

When our brain's thinking cells, called neurons, start oozing too much of the amyloid protein that is the hallmark of Alzheimer's disease, the brain's helper cells, called astrocytes, that normally nourish and protect them deliver a suicide package instead, the researchers report.

Amyloid is excreted by all neurons, but rates increase with age and dramatically accelerate in Alzheimer's. Astrocytes, which deliver oxygen and nutrients to neurons in addition to hauling off some of their garbage, get activated and inflamed by excessive amyloid. [Living With Alzheimer's in the U.S. (Infographic)]

"If the neuron makes something toxic and dumps it at your door, what would you do?" Study researcher Erhard Bieberich, of the Georgia Health Sciences University, said in a statement. "You would probably do something to defend yourself."

The researchers found that these astrocytes "defend themselves" by packaging together a deadly duo of proteins and sending them into the neuron.

The study was published April 24 in the Journal of Biological Chemistry.

The researchers hypothesize that this package ultimately kills both the neuron and the astrocyte, which could help explain the brain-cell death and shrinkage that occurs in Alzheimer's.

"If the astrocytes die, the neurons die," Bieberich said. Earlier studies have suggested that excess amyloid alone does not kill brain cells — and the recent work suggests that the cell's reaction to amyloid that causes brain cell death.

"There must be a secondary process toxifying the amyloid; otherwise the neuron would self-intoxicate before it made a big plaque," Bieberich said. "The neuron would die first."

The researchers think that by attacking this deadly package sent from the astrocyte to the neuron, they could find some way to treat Alzheimer's. In the researchers' studies of brain cells of humans with Alzheimer's as well as an animal model of the disease, antibodies to the proteins in this deadly duo prevented astrocytes' amyloid-induced death.