The protein, tumour necrosis factor (TNF), triggers a self-destruct mechanism in diseased or mutated cells. The process is called apoptosis.

However, scientists in the US suspect that it also weakens the immune system as people grow older.

The study provides useful clues as to how the ageing process works.

The report appears in New Scientist magazine.

Immune cells die

"Researchers in California say that as the body ages, protective cells of the immune system become hypersensitive to the protein, and die at high rates," the report said.

Researchers at the University of California, Irvine, studied immune cells known as CD4 and CD8 in blood samples.

The samples came from young and old subjects - students and retired professors.

The report said: "When exposed to TNF, 26% of the CD8 cells from the young subjects underwent apoptosis. By contrast, 40% of the aged CD8s committed suicide."

A similar increase in cell death among the older group's CD4 cells was also found.

The researchers, led by immunologist Dr Sudhir Gupta, concluded they had become more sensitive to TNF.

Dr Gupta said there is more to the weakening of the immune system than sensitivity to TNF, but added that the protein could turn out to be an important factor.

He told the magazine that drugs could one day slow the ageing process.