Prions naturally present in the brain appear to prevent the build up of a key protein associated with the condition.
In laboratory tests, beta amyloid, the building block of Alzheimer's "plaques", did not accumulate if high levels of the prions were present.
The findings could lead to new treatments, the Proceedings of the National Academy of Sciences reported.
In variant Creutzfeldt-Jakob disease (vCJD), the human version of mad cow disease, the normal version of the prion protein present in brain cells is corrupted by infectious prions causing it to change shape, resulting in brain damage and death.
But little is known about purpose of the normal prion proteins.
Due to the similarities between Alzheimer's and diseases such as variant CJD, researchers at the University of Leeds, looked for a link.
Plaque formation
They found that in cells in the laboratory, high levels of the prions reduced the build-up of beta-amyloid protein, which is found in the brains of people with Alzheimer's disease.
In comparison, when the level of the prions was low or absent, beta amyloid formation was found to go back up again, suggesting they have a preventive effect on the development of the condition.
The researchers also looked at mice who had been genetically engineered to lack the prion proteins and again found that the harmful beta-amyloid proteins were able to form.
Study leader Professor Nigel Hooper said they now needed to look at whether ageing had an affect on the ability of the prion proteins to protect against Alzheimer's.
"Until now, the normal function of prion proteins has remained unclear, but our findings clearly identify a role for normal prion proteins in regulating the production of beta-amyloid and in doing so preventing formation of Alzheimer's plaques.
"Whether this function is lost as a result of the normal ageing process, or if some people are more susceptible to it than others we don't know yet."
He said although they needed to learn more, theoretically if a treatment could be designed to mimic the effect of the prions it could halt the progression of the disease.
Professor Clive Ballard, director of research at the Alzheimer's Society said this was the first time a link had been made between prions and Alzheimer's.
"These are early findings, which suggest prion proteins may have a regulatory effect on the development of beta amyloid."
He added: "This provides the foundations for a novel approach to finding new therapeutic targets in Alzheimer's disease."