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Frequent blood donors may be getting more than a warm, fuzzy feeling from their altruism, as giving blood may also enhance your ability to produce healthy blood cells, potentially reducing the risk of developing blood cancer.

at the Francis Crick Institute in London and his colleagues analysed genetic data extracted from blood cells donated by 217 men in Germany, aged between 60 and 72, who had given blood more than 100 times. They also looked at samples from 212 men of a similar age who had donated blood fewer than 10 times, and found that frequent donors were more likely to have blood cells carrying certain mutations in a gene called DNMT3A.

To understand this difference, the team genetically engineered human blood stem cells – which give rise to all blood cells in the body – with these mutations and added them to lab dishes along with unmodified cells. In order to mimic the effects of blood donation, they also added a hormone called EPO, which the body produces following blood loss, to some of the dishes.

A month later, the cells with the frequent-donor mutations had grown 50 per cent faster than those without the mutations, but only in the dishes containing EPO. Without this hormone, both cell types grew at a similar rate.

“It suggests that, every blood donation, you’re going to have a burst of EPO in your system and this is going to favour the growth of cells with these DNMT3A mutations,” says Encabo.

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To investigate whether having more of these mutated blood cells is beneficial, the team mixed them with cells carrying mutations that raise the risk of leukaemia, and again found that, in the presence of EPO, the frequent-donor cells substantially outgrew the others, and were better able to produce red blood cells. This suggests that the DNMT3A mutations are beneficial and might suppress the growth of cancerous cells, says Encabo.

“It’s like the donation of blood is providing a selection pressure to enhance the fitness of your stem cells and their ability to replenish,” says at the University of Bristol, UK. “Not only could you save someone’s life, but maybe you are enhancing the fitness of your blood system.”

Further work is needed to verify if this is really the case, says at University College London, as lab experiments provide a highly simplified picture of what happens in the body. “This needs to be validated in a much larger cohort, across different ethnicities, across females and other age groups,” says Mansour. He also points out that donors without the DNMT3A mutation may not see this benefit.