What are blood types?

Every drop of blood contains red blood cells, which carry oxygen throughout your body. It also contains white blood cells, which help fight infection, and platelets, which help your blood clot.

But that’s not where it ends. Your blood also contains antigens, which are proteins and sugars that sit on red blood cells and give blood its type. While there are at least 33 blood typing systems, only two are widely used. These are the ABO and the Rh-positive/Rh-negative blood group systems. Together, these two groups form the eight basic blood types that most people are familiar with:

  • A-positive
  • A-negative
  • B-positive
  • B-negative
  • AB-positive
  • AB-negative
  • O-positive
  • O-negative

Keep reading to learn more about blood types and why it’s hard to say which type is the rarest in the world.

What determines blood type?

Blood types are determined by genetics. You inherit genes from your parents — one from your mother and one from your father — to create a pair.

ABO system

When it comes to blood type, you might inherit an A antigen from one parent and a B antigen from the other, resulting in the AB blood type. You could also get B antigens from both parents, giving you a BB, or a B, blood type.

Type O, on the other hand, doesn’t contain any antigens and has no effect on A and B blood types. This means that if you inherit an O from your mother and an A from your father, for example, your blood type would be A. It’s also possible that two people with type A or type B blood could have a baby with type O blood if they carry the O antigen. For example, parents with AO blood could each pass the O antigen on to their child, creating OO (or simply O) blood. There are six of these combinations (AA, AB, BB, AO, BO, OO), which are called genotypes. The four blood types (A, B, AB, and O) stem from these genotypes.

Rh factor

Blood is also typed according to something called the Rh factor. This is another antigen found on red blood cells. If the cells have the antigen, they’re considered Rh-positive. If they don’t have it, they’re considered Rh-negative. Depending on whether the Rh antigen is present, each blood type is assigned a positive or negative symbol.

What’s the rarest blood type?

It’s hard to say which blood type is the rarest in the world, since they’re linked to genetics. That means the prevalence of certain blood types varies widely in different parts of the world.

However, in the United States, AB-negative is considered to be the rarest blood type, and O-positive the most common. The Stanford School of Medicine Blood Center ranks blood types in the United States from rarest to most common as follows:

  1. AB-negative (.6 percent)
  2. B-negative (1.5 percent)
  3. AB-positive (3.4 percent)
  4. A-negative (6.3 percent)
  5. O-negative (6.6 percent)
  6. B-positive (8.5 percent)
  7. A-positive (35.7 percent)
  8. O-positive (37.4 percent)

Again, this ranking isn’t universal. In India, for example, the most common blood type is B-positive, while in Denmark it’s A-positive. These variations also exist within groups of Americans. According to the Red Cross, for instance, Asian Americans are much more likely to have a B-positive blood type than Latin Americans and Caucasians.

Why blood type matters

Your immune system naturally contains protective substances called antibodies. These help to fight off any material that your immune system doesn’t recognize. Usually, they attack viruses and bacteria.

However, antibodies can also attack antigens that aren’t present in your natural blood type. For example, if you have type B blood that’s mixed with type A blood during a transfusion, your antibodies will work to destroy the A antigens. This can have life-threatening results, which is why medical centers around the world have strict procedures in place to keep this from happening.

Keep in mind that blood types don’t always need to be an exact match to be compatible. For example, AB blood has both the A and B antigen, so a person with this type of blood can receive either type A or type B blood. Everyone can receive type O blood because it doesn’t contain any antigens. This is why people with type O blood are considered “universal donors.” However, people with type O blood can only receive type O blood.

When it comes to the Rh factor, people with Rh-positive blood can receive either Rh-positive or Rh-negative blood, while people with Rh-negative blood can only receive Rh-negative blood. In some cases, a woman with Rh-negative blood can carry a child with Rh-positive blood, resulting in a dangerous condition called Rh incompatibility.

The bottom line

While everyone’s blood generally looks the same, there’s a complex set of systems used to categorize what goes on beneath the surface. There are dozens of blood typing systems, but most people are familiar with the ABO and Rh systems, which provide the eight basic blood types. Generally, AB-negative is considered to be the rarest blood type. However, because blood type is linked to genetics, there is no single type that’s considered to be the rarest worldwide.