The work may allow doctors to test the strength of a patient's immune system and pick out those who are destined to relapse long before it becomes a clinical problem.

The measurement of a patient's immune response to their leukaemia when they have completed their therapy may be used as an indicator or risk of subsequent relapse

Dr Mark Lowdell

Giving these patients treatment to boost their immune system - possibly by a vaccine - could ultimately save their lives, the researchers suggest.

Dr Mark Lowdell, a Leukaemia Research Fund-supported scientist at the Royal Free Hospital, London, said: "Our findings propose that it is the immune response, rather than the chemotherapy per se, which is responsible for continued remission.

"The measurement of a patient's immune response to their leukaemia when they have completed their therapy may be used as an indicator or risk of subsequent relapse."

The researchers studied 25 leukaemia patients who went into remission following chemotherapy or bone marrow transplantation.

Killed cells

They assessed how powerfully the patients' immune systems responded to leukaemia cells - by mimicking the encounter between immune cells and the cancer cells.

They then calculated the percentage of leukaemia cells that were successfully killed.

Dr Lowdell believes this proportion has to be above a certain threshold to keep patients in remission.

The patients were monitored for more than eight years, and the researchers found that all patients who relapsed had immune responses below the threshold.

All of the adult patients and half of the children who remained in remission had anti-leukaemia responses that exceeded the threshold.

The researchers conclude that the immune system plays a vital role in keeping cancer at bay.

Dr Lowdell said: "Initially leukaemia overwhelms the immune system, making it impossible for it to fight back, and the patient gets sick.

"We can counter the attack with chemotherapy that devastates the diseased cells but it probably doesn't eradicate them completely.

"Some patients then appear to make an immune response to the leukaemia.

Early stage

"It is clear from our research that the strength of this counter attack by the immune system is absolutely vital to the patient's sustained good health.

"These are preliminary results, but they are very exciting.

"If the results are replicated in larger-scale studies we will have a simple, easy-to-perform test to accurately predict whether patients will remain in remission or whether they are likely to relapse."

The likely treatment at present for patients facing relapse is a bone marrow transplant, but as research in this area continues Dr Lowdell believes it will be possible to stimulate their immune systems to kill the cancer cells.

Dr David Grant, scientific director of the Leukaemia Research Fund, said: "We have known for some time that leukaemia is vulnerable to cells from the immune system, but never has it been reported so clearly or linked so closely to relapse.

"Learning to harness this is the next challenge and work is already underway.

"As our knowledge of the immune system unfolds we hope to be able to switch on its cancer-killing capabilities as and when required."

The research is published in the British Journal of Haematology.