Blood Types

Blood types refer to the classification system that healthcare providers use to ensure you receive blood that’s a good match when you need a blood transfusion or an organ transplant.

There are four main blood types: A, B, AB and O. Blood bank specialists determine your type based on whether you have antigen A or B on the surface of your red blood cells. They also look for a protein called the Rh factor. They classify your blood as positive (+) if you have this protein and negative (-) if you don’t. All told, there are eight common blood types:

• A positive (A+)
• A negative (A-)
• B positive (B+)
• B negative (B-)
• AB positive (AB+)
• AB negative (AB-)
• O positive (O+)
• O negative (O-)

How do healthcare providers determine blood types?

Transfusion specialists identify your type by checking your red blood cells for specific surface antigens. Antigens are markers, like a nametag, which let your immune system know if a specific antigen belongs in your body.

There are two antigen systems that specialists check: the ABO system and the Rh system. The ABO system classifies types by whether your red blood cells have the A antigen or the B antigen. Types in the ABO system are:

• Type A: Red blood cells have the A antigen.
• Type B: Red blood cells have the B antigen.
• Type AB: Red blood cells have both A and B antigens.
• Type O: Red blood cells have neither A nor B antigens.

There are over 50 inherited antigens within the Rh system. The blood bank checks for the RhD antigen. Red cells are classified based on whether they have the RhD antigen:

• RhD positive (+): Red blood cells have the RhD antigen.
• RhD negative (-): Red blood cells don’t have the RhD antigen.

Your blood is compatible with someone else’s if your immune system recognizes the antigens in donated blood cells. That recognition keeps your immune system from attacking donated cells.

What’s the most common blood type?

O positive (O+) is the most positive type. The least common one is AB negative (AB-).

What are rare blood types?

A rare blood type is defined as one occurring in only 1 out of every 1,000 people. The Rh null blood type, in which all of the Rh antigens are missing, is one example. Fewer than 50 people in the world have this type. Rh null blood is sometimes called “golden blood” because it’s so rare, and it can be used in blood transfusions for people with rare Rh blood types.

There are more than 600 antigens outside the ABO and Rh systems that may be present on red blood cells. These unique antigens relate to more than 30 different blood group systems, including the:

• Duffy blood group
• K antigen (or Kell) group
• Lutheran blood group
• Kidd blood group

How do people inherit their blood type?

You inherit it the same way you inherit your eye color — from your biological parents. But you may not inherit the exact ones that your parents have. That’s because you inherit both the ABO genes and the Rh factor genes. The various antigen and protein genes can create many different combinations that make up your blood type.

Which blood types are compatible?

An important part of blood donation is ensuring you receive donated blood from someone whose blood type is compatible with yours. That means their red blood cells contain antigens that your body recognizes. This is critical because your immune system may attack red blood cells that have antigens that don’t match yours.

Here’s information about compatible types. But keep in mind that all the less common non-ABO blood group antigen systems need to be compatible.

What is a universal donor?

A universal donor is someone with O negative (O-) blood. That person can donate blood to anyone. In an emergency transfusion, healthcare providers typically use type O negative blood when they don’t know the blood type of the person who needs a transfusion.

What is a universal recipient?

This is someone with blood type AB positive (AB+). If this is your blood type, you can safely receive blood from anyone, regardless of their blood type.

How do I find out my blood type?

Your healthcare provider may have this information on record. And there are at-home blood test kits you can buy.

If you’re able, donating blood is another way to learn your blood type. Blood bank specialists identify your blood type by mixing your blood sample with antibodies that attack antigens A or B. The results show which antigens are on your red blood cells. 

Can my blood type change?

Very rarely, it may change if you have a stem cell transplant. Certain types of leukemia or infections may change it. And the change may not be permanent.

How does blood type affect pregnancy?

A fetus may inherit its Rh factor (either positive or negative) from either parent. Your obstetrician will check your blood type and screen for antibodies as part of your first prenatal appointment. If you have Rh negative blood and the fetus has Rh positive, there’s a chance your immune system will attack the fetus’s red blood cells. If tests show a mismatch, you’ll receive medication that blocks the attack.