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Healthy Living · article

Should I know my blood type?

Blood tests

From pregnancy to blood transfusions to blood donations, there are many situations where your blood type is a critical piece of information in your care – and the care of others.

Sreevani Thota, MD, a primary care physician with Atlantic Health System, shares more about blood types and why it helps to know yours. 

What are blood types?

Your blood type is a combination of antigens, or markers, on your red blood cells, as well as an Rh factor, a different marker that can be found on the cell’s surface. You inherit both your antigen and Rh factor from your parents. There are four antigen combinations (A, B, AB and O). The Rh factor can either be positive, which means the protein is present on the cell’s surface, or negative when it isn’t.

With these combinations, there are eight common blood types:

  • A+
  • A-
  • B+
  • B-
  • AB+
  • AB-
  • O+
  • O-

In addition to these groups, there are many rare blood groups that do not fall under the ABO system. Most Americans have a positive Rh factor. The most common blood type in the U.S. is O+, while the least common is AB-.

You can find out your blood type during your next blood test in your doctor’s office or when you donate blood.

What blood types are compatible?

It’s important to know if your blood type is compatible with other blood types. That’s because your immune system will attack blood cells with antigens they don’t recognize, which can be very dangerous.

“Knowing your blood type is important if you are pregnant or if you find yourself in an emergency situation,” says Dr. Thota. “For example, receiving blood with the wrong antigen or a positive Rh factor if you are negative can be life-threatening. Having that information available can help your care team provide the right blood for you, quickly.”

Compatible blood types include:

  • A+: A+, A-, O+, O-
  • A-: A-, O-
  • B+: B+, B-, O+, O-
  • B-: B-, O-
  • AB+: A+, A-, B+, B-, AB+, AB-, O+, O- (universal recipient)
  • AB-: A-, B-, AB-, O-
  • O+: O+, O-
  • O-: O- (universal donor)

How are blood types used?

Knowing your blood type – and what other types are compatible — can be important in different situations including:

  • Blood donation: While blood donations of all types are nearly always needed, your local facility may need a particular blood type more than others. Donating blood can help save a life.
  • Blood transfusions: In cases including emergencies, surgeries or other major medical events, you may need to receive blood from a donor. If that blood is not compatible, it can cause dangerous complications.
  • Pregnancy: If a pregnant patient has a negative Rh factor and their fetus has a positive Rh factor, the mother’s immune system may try to attack the red blood cells of the fetus. Fortunately, expectant mothers are screened early in pregnancy and a simple shot called RhoGAM helps stop the immune system from responding to these cells.
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Published: February 12, 2025

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