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They say it does that by increasing the number of red blood cells that contain haemoglobin, which transports oxygen around the body, but this remains a disputed subject and many say further studies are required.

And when mice were injected with these human blood stem cells in the lab, the cells were found to be good at making red blood cells - a positive sign, Dr Encabo said.

"You are not just helping people with your red blood cells – now the plasma in your blood donation is helping people too."

The condition occurs at pregnancy when the mother's red blood cells are incompatible with that of their growing baby.

It is caused by genetic change that means people make haemoglobin - a key protein in red blood cells - that doesn't work properly.