They say that the levels of the antibodies found in healthy people decline with age and, in Alzheimer's patients, with increasing progression of the disease.
Alzheimer's disease is characterized by the build-up of amyloid plaques in the brain.
These are large aggregations of a protein breakdown product, or peptide, called A-beta. Many experiments have shown that immunization with A-beta can reduce the formation of amyloid plaques.
It is believed that more than the plaques themselves (which are also found in the brains of people with no Alzheimer's symptoms), it is smaller aggregations of a few A-beta molecules, called oligomers, which are most toxic to neurons.
During the study, the researchers found that the antibodies target many forms and aggregation-states of A-beta in both healthy and diseased subjects' blood, with antibodies to oligomers showing the most immunoreactivity.
A follow-on experiment showed that the same antibodies, whether isolated from plasma of either Alzheimer's patients or healthy controls, were able to protect freshly cultured mouse neurons in a dish from destruction by A-beta, which is typically highly toxic to these neurons.
Previous studies conducted on vervet monkeys showed that immunizing with A-beta substantially cleared their plaques.
In this new study, the Stanford team obtained blood samples extracted from those monkeys before and after immunization, and compared levels and diversity of relevant antibodies in pre- and post-inoculation samples
They observed several such antibodies in the pre-immunized samples, as well as significant post-immunization increases in levels of several different antibodies.
"Other studies have found antibodies against A-beta, but nobody has ever done a large-scale analysis using hundreds of different samples and almost a hundred different peptides to look for what's already in people's bodies," said the paper's first author, Markus Britschgi, PhD, an instructor working as a researcher in the laboratory of Tony Wyss-Coray, PhD.
The study appears in the journal Proceedings of the National Academy of Sciences.