Which human blood type is the universal donor?

O negative, because its red cells carry neither the A nor B markers nor the Rhesus D antigen, so there is nothing on them for a recipient's immune system to attack. It is what ambulances and trauma bays carry when there is no time to type a patient - and it is chronically in short supply, because only about seven per cent of people have it.

It runs the other way for plasma. A person with AB blood has no anti-A or anti-B antibodies circulating, so AB plasma can go to anyone - the universal donor for cells and the universal donor for plasma are opposite ends of the same system.

The whole thing was unknown until 1901, when Karl Landsteiner worked out why some transfusions killed people and others did not. Before that, transfusion was a lottery that had been tried, banned across Europe after deaths in the 1660s, and revived in the nineteenth century with poor results. Researchers are now working on enzymes from gut bacteria that strip the A and B markers off donated cells, which would in principle convert any blood into something close to universal.