The universal blood type is the rarest and most desired in a hospital setting. Universal donors are called this because they can donate to anyone without prior testing, as long as they meet certain criteria. Hospitals need to have these people on staff (or at least ready) because their availability makes it possible for everyone who needs transfusions to get one.

Blood types

The first universal donor was found in 1937, and it took until the 1980s for more research to be done on this type of blood. In 1986, a man who had been donating his blood discovered that he met all of the criteria and became known as O negative or O-. All other types are called A+, A- or B+. 

The universal blood type is rare because only about 11% of the population has this blood. However, they are always in demand and hospitals try to keep a high number of stocks at all times. Those who have O-type blood can donate as often as every 56 days; other types must wait 12 weeks before donating again.

Blood transfusions:

The blood type of the transfusion recipient (A+, A- or B+) dictates which universal donor can donate to them. However, there are always exceptions and people must also be compatible with one another in other ways before a transfusion is given.

If an O+ person needs a red blood cell transfusion, they need about double the volume of red cells than an A- person. But no matter what type of universal donor is chosen, they are always compatible with each other and will never end up fighting over available blood in a hospital setting.

Universal blood recipients and donors:

After the discovery of universal donors, it was found that transfusions were not as necessary as they once had been. Blood banks became more about storing blood for emergencies and surgeries than giving it out to those in need. The original intention is still there though -- hospitals always want an on-site supply of O+ type universal donor blood ready at all times. They do this by recruiting donors who will be able to give blood when needed.

Universal plasma donors:

Plasma is an important component of blood that can't be stored for long periods. That's why it must come from a living donor, and the process is different than donating whole blood. The universal plasma donors are O- type people who have to donate every 56 days because their plasma has more antibodies in it which help protect against possible transfusion reactions.

Another type of blood that can be donated is platelets, which are a component in the blood that help with clotting. Platelet donors must be O- or A+ to donate and they only have to wait 12 weeks before donating again because their plasma doesn't contain as many antibodies.

The Bottom Line:

It's important to know your blood type and that of the person you're about to have a transfusion with, as well as compatibility in other ways. There are plenty of cases where someone needs a transfusion but can't get one because there aren't any donors nearby or they don't match up in their requirements for giving whole blood.