A blow to the head that knocks you out can raise your chance of getting Alzheimer’s disease someday, but only if you carry a particular gene, a study suggests.

Elderly people who had suffered a severe blow to the head and also carried the gene were 10 times as likely to have Alzheimer’s as were people with neither risk factor.

However, a head injury alone, without the gene, did not raise the risk.

The gene might act on a head injury by turning a normal repair process into a step toward disease, said Dr. Richard Mayeux, a professor of neurology, psychiatry and epidemiology at Columbia University in New York.

Head injury and the gene, apo-E4, each had been linked separately to Alzheimer’s risk in prior studies.

Mayeux and colleagues published the new results in this month’s issue of the journal Neurology.

“It’s actually a very important paper” because it sheds light on how head injury promotes Alzheimer’s, said Dr. James Mortimer, associate director of the Geriatric Research, Education and Clinical Center at the Veterans Affairs Medical Center in Minneapolis.

Walter Kukull, an epidemiologist at the University of Washington in Seattle who is studying Alzheimer’s, cautioned that the conclusions must be considered tentative because relatively few study participants had a head injury plus the gene.

But the idea that apo-E4 could team up with a head injury to raise the risk of Alzheimer’s is plausible, he said.

Although head injuries would account for only a small fraction of Alzheimer’s cases, the new research is valuable for its insight into how genes and environment might work together to produce the disease, Kukull said.

The study included 113 Alzheimer’s patients and 123 healthy elderly people who were matched to the patients by age, gender and ethnic group.