Only one in every six million people have the Rh null blood type. Now researchers are trying to grow it in the laboratory in the hope it could save lives.

Blood transfusions have transformed modern medicine. If we are ever unlucky enough to be injured or need serious surgery, blood that has been donated by others can be life-saving.

But not everyone is able to benefit from this remarkable procedure. People with rare blood types struggle to find donated blood that will match their own.

One of the rarest – the Rh null blood type – is found in just 50 known people in the world. Should they ever be in an accident that needs a transfusion, their chances of getting one are slim. Those with Rh null are instead encouraged to freeze their own blood for long-term storage.

But, despite its rarity, this blood type is also highly prized for other reasons. Within the medical and research community it is sometimes referred to as "golden blood" due to how it can be used.

It may also help to create universal blood transfusions as scientists search for ways of overcoming the immunity issues that currently restrict how donated blood is used.

The type of blood you have circulating around your body is classified based on the presence or absence of specific markers on the surface of your red blood cells. These markers, known as antigens, consist of proteins or sugars which stick out from the cell surface and can be detected by the body's immune system.

"If you get transfused with donor blood that contains different antigens to your own blood, you'll make antibodies to that blood and attack it," says Ash Toye, professor of cell biology at the University of Bristol. "If you get transfused with that blood again, it can be life threatening."

The two blood group systems that evoke the largest immune response are ABO and Rhesus (Rh). A person with an A blood group has A antigens on the surface of their red blood cells, while someone with a B blood type has B antigens. The AB blood group has both A and B antigens, with the O group has neither. Each group can be either Rh positive or Rh negative.

People with O negative blood are often described as universal donors, as their blood contains neither A, B, or Rh antigens. However, this is an oversimplification.

First, there are currently 47 known blood groups and 366 different antigens, as of October 2024. That means that a person receiving an O negative donation could still have an immune reaction to any of the other antigens present – although some antigens provoke more of an immune response than others.

Secondly, there are over 50 Rh antigens. When people talk about being Rh negative they are referring to the Rh(D) antigen, but their red blood cells still contain other Rh proteins. There is also a huge diversity of Rh antigens across the world, making it challenging to find true donor matches, especially for people from ethnic minority backgrounds in a given country.

People with Rh null blood, however, lack all 50 Rh antigens. While these people cannot receive any other blood........

© BBC