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Vast numbers of cells that can attack cancer and HIV have been grown in the lab, and could potentially be used to fight disease.
The cells naturally occur in small numbers, but it is hoped injecting huge quantities back into a patient could turbo-charge the immune system.
The Japanese research is published in the journal Cell Stem Cell.
Experts said the results had exciting potential, but any therapy would need to be shown to be safe.
The researchers concentrated on a type of white blood cell known as a cytotoxic T-cell, which can recognise telltale markings of infection or cancer on the surfaces of cells. If a marking is recognised, it launches an attack.
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