Giving blood, serving in the military or needing a transfusion are a few ways people learn their blood type. And what they will likely hear is a combination of A or B or O, positive or negative.

What almost no one hears is Rh-null, the world’s rarest blood type, which has only been identified in fewer than 50 people worldwide.

Blood types are determined by antigens, which are the proteins and sugars that are attached to red blood cells.

The eight basic blood types are A+, A-, B+, B-, AB+, AB-, O+, and O-, with the positive or negative determined by the presence or absence of another antigen, the RhD protein.

And some 342 different antigens exist, 160 of which are highly prevalent. These antigens are organized into 35 blood group systems, the biggest being the Rh system with 61 antigens.

Not having one of the prevalent antigens make a blood type rare. But Rh-null blood lacks antigens altogether in its Rh system and is found in roughly one in six million people. Rh-null was first reported in 1961, found in an Aboriginal woman in Australia.

“It’s the golden blood,” Dr. Thierry Peyrard, director of the National Immunohematology Reference Laboratory in Paris, told Mosaic Science.

“You would not imagine how difficult it is when you have to import or export rare blood.”
Dr. Thierry Peyrard

Lacking its antigens, Rh-null is quite valuable as a universal donor type for those with Rh blood systems — but only nine active donors are known to exist.

And of course, those who carry this rare blood are at risk should they ever need a blood donation. One of them, a middle-aged Swiss man, has been featured in media reports describing how careful he must be. For example, he doesn’t travel to countries that lack modern medical facilities, and he drives very carefully to avoid injuries and accidents.

“You would not imagine how difficult it is when you have to import or export rare blood,” Peyrard told The Atlantic. “Your patient is dying, and you have people in an office asking for this paper and that form. It’s just crazy. It’s not a TV set, it’s not a car. It’s blood.”

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