Male hospital patients receiving blood are more likely to die if they receive transfusions from a woman donor who has been pregnant, a new study suggests.

Researchers believe that antibodies acquired by women during pregnancy to protect the baby may trigger reactions in men which could prove fatal.

The most common cause of death was transfusion-related acute lung injury (Trali). Although the scientists warn that more studies are needed to replicate the findings, they say the results ‘if true, have significant clinical implications.’

Around two million blood transfusions are given in Britain each year and recipients are not told if their donor was a man or a woman.

Writing in the Journal of the American Medical Association, Dr Rutger Middelburg, from Sanquin Research in Leiden, and colleagues said: “Male recipients who received a transfusion from an ever-pregnant female donor had a statistically significant increase in mortality compared with those who received a transfusion from a male donor or from a female donor without a history of pregnancy.

“The association of increased mortality among male patients who received transfusions from ever-pregnant donors suggests a possible mechanism based on immunologic changes occurring during pregnancy.

“An alternative explanation could be a difference in iron status between ever-pregnant female and male donors. Some studies also report differences in red blood cell physiology between the sexes.