Researchers are starting to understand more about the early signs of Alzheimer's disease.

There are certain patients who could be the key to helping researchers learn even more about the disease.

"The postmortem diagnosis it was 'oh, this person had Alzheimer's,' but when you see, and you go to the clinical records, they were perfectly normal," Indiana University School of Medicine Assistant Research Professor Nur Jury-Garfe said.

Now researchers are trying to determine how these asymptomatic cases avoid cognitive decline. One theory involves immune cells in the brain called microglia.

"You have the plaque, and then in the asymptomatic cases the microglia is more like dynamic. So it can reach faster the plaque. Once it's there in the plaque, it can embrace the plaque and start fighting and eating all these toxic molecules," Jury-Garfe said.

If researchers can find a way to mimic these protective mechanisms, it could lead to therapies to help slow the progression of symptoms in someone with the disease.

According to the National Institutes of Health, 30 to 50 percent of brains donated to research as "controls", or healthy, end up having these markers of Alzheimer's.

The older individuals, with an average age of 85, had no symptoms of the disease while living.