Antibody therapy is used to treat cancer
An immune system therapy given to cancer patients could have the added benefit of reducing the risk of Alzheimer's disease, a study suggests.
A US team found patients who had received antibody treatment had more than 40% less risk of Alzheimer's than people who had not.
Writing in Neurology, they said a bigger study was needed to confirm their findings.
UK experts said immunotherapy was an important area of research.
So far, scientists have been looking at it as a way of treating people who already have Alzheimer's.
The idea is that immune based therapies affect the formation of beta-amyloid plaques in the brain, which are characteristic of Alzheimer's, possibly by suppressing the inflammatory response in the brain.
People with the disease have lower levels of anti beta-amyloid antibodies, so experts are looking at ways of boosting levels - including immunisation.
But this study investigated whether or not people who had been given the treatment already, for another condition, had some protection.
'Treat the cause'
The team from Mount Sinai School of Medicine in New York looked at the records of 847 people who had been given at least one intravenous immunoglobulin
(IVIg) treatment for cancers, such as leukaemia, or immune system disorders.
All were over 65 and had received the treatment between April 2001 and August 2004.
Their records were then compared with those of 847,000 people who had not needed the therapy who were similar Alzheimer's risk factors to the treated group.
The records were held by a medical insurance company, and so detailed the illnesses and treatments people had claimed payments for.
Patients were followed up to August 2007. It was found that only 2.8% of those treated with IVIg developed Alzheimer's, compared with 4.8% of those not treated.
Dr Howard Fillit, who led the study, said: "IVIg has been used safely for more than 20 years to treat other diseases but is thought to have an indirect effect on Alzheimer's disease by targeting beta-amyloid, or plaques in the brain.
"Our study provides evidence that previous IVIg treatments may protect against Alzheimer's disease.
"The current Alzheimer's drugs on the market treat the symptoms of the disease. Immunization could treat the underlying cause."
But he added: "These findings do not constitute an endorsement of IVIg treatment for Alzheimer's disease. A large scale clinical trial is underway to determine whether IVIg could be an effective treatment for Alzheimer's."
Neil Hunt, chief executive of the Alzheimer's Society, said: "This is a really encouraging epidemiological study.
"Clinical trials are now underway in this area and we look forward to the results."
But he added: "Introducing large amounts of antibodies could cause serious side effects so important questions will need to be answered before this treatment becomes available."