A, B and O.

These are the most common blood types. The ones we all know. Then there's one called the Bombay Blood group which is considered extremely rare. It was discovered in Mumbai in 1952, hence the name, and is said to show up in 4 people out of a million.

Now that's rare. But what if there was one which was even rarer?

Source: rvdoon

Introducing, Rhnull or the cooler name - Golden Blood. 

Why is it called so? Because it is the rarest blood type in the entire world. So rare that only around 40 people are recorded to have it and there are only 9 donors in the world.

Source: bloodjournal

We all have red blood cells and those cells have around 342 antigens. These antigens produce antibodies. A person's blood type is determined on the basis of the presence or absence of these antigens.

160 of 342 blood group antigens are prevalent. Meaning, most people have it. Your blood type is considered rare if you lack antigens that 99% of the people are positive for. If you somehow lack an antigen that 99.99% are positive for, your blood type is extremely rare.

Source: faranworld

In 1974, ten-year-old Thomas went to the University Hospital of Geneva to get an infection checked out. 

There are 35 recognised blood group systems. Thomas didn't seem to have any. Forget negative or positive, he had no Rh antigens to speak of. Technically, he shouldn't have been alive. Shocked, the doctor sent him to Amsterdam and Paris for further blood tests. It was confirmed. Thomas had Rhnull blood type.

Source: centralfm

The thing about Rhnull is that if you have it, get ready to be approached by scientists and doctors alike. Purely because they want to study it further. Rhnull can be the true universal donor group for anyone who has a rare blood type. It's life saving capability is humongous.

Source: wbur

Those who have it, have the power to save lives. But should something happen to them, they could be in grave danger. And mixing blood types can be lethal. Since there are so few rare blood types. 

If there is blood required for a very rare blood type in a different part of the world, you can't really send it across borders. Take Thomas for example. He actually flies to France to donate due to strict laws. Also, he isn't paid for it. He does so on his own dime. 

Source: cari-toko

He first did it when a baby in Geneva needed it.

People with Rhnull blood type can have children and lead a normal family life. They can be mildly aneamic, but that's about it.

But life is different for them. They need to extra careful. Thomas doesn't travel to countries that don't have modern medical facilities. He carries a card that confirms his Rhnull status, drives very carefully and most of all, he avoids injuries and accidents.

Source: mosaicscience

Apparently there are 8 others who donate. But it's hard to say how active they are. Goes without saying, it is the most sought after donor blood group. 

For that very reason, doctors hold it in high esteem. They use it only for very very extreme and rare cases. Because once used, it is almost impossible to replace. Hence, Golden Blood!