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What does your blood type mean for your health? By Donna Christiano

If all you know about your blood is that it’s red, you have some catching up to do.

Blood is made up of a lot of different components. There are red and white blood cells, which carry oxygen and help fight infection, respectively. There are platelets, which help your blood clot. And there’s plasma, which provides the body with things like nutrients and hormones. Your plasma contains antibodies, which are substances your immune system uses to fight foreign invaders like germs and bacteria.

Your blood also contains antigens. These are proteins and other molecules present on the outside of your red blood cells; they determine what type of blood you have. Blood is further classified by its rhesus factor (aka, Rh factor). If your blood contains the Rh D factor—the most prevalent and important of the Rh factors—you have a positive blood type. If your blood lacks it, you have a negative blood type. 

Categorizing blood according to type is important for things like blood transfusions, which replace blood that’s lost through surgery, accidents, and bleeding disorders. Mixing one type of blood that’s incompatible with another—thanks to things like antigens and Rh factor—can be fatal. 

Healthy blood is essential for a healthy life. From typing to transfusing, here’s what you need to know about your blood and your health.

How many blood types are there?

The vast majority of people have one of eight blood types. Again, blood types are based on the antigens (or lack of them) found on your blood cells and whether or not your blood contains the Rh D factor. Blood is typed according to an ABO blood group system. If your blood has A antigens, you have an A blood type. If you have B antigens, you have a B blood type. Some people have both A and B antigens, giving them AB blood. And people with an O blood type have neither A nor B antigens. 

Each of those types are further broken down based on their Rh factor. For example, some people have A positive blood while others have A negative. A very small number of people have what’s called Rh null blood (also called gold blood), meaning it has no Rh factors at all. This is extremely rare, occurring in only a handful of people worldwide. 

How common or rare a blood type is varies by race, ethnic background, and what part of the world you live in. According to the book Blood Groups and Red Cell Antigens, blood type B is common in people in Asia while blood type A is common in Central and Eastern Europe. In the U.S. and Western Europe, O positive is the most common blood type, as is having a positive Rh factor. AB negative is the rarest. What about the rest of the blood types? The Stanford Blood Centerprovides these statistics.

Blood typePercentage of Americans with blood type
O+37.4%
A+35.7%
B+8.5%
O-6.6%
A-6.3%
AB+3.4%
B-1.5%
AB-0.6%

What’s my blood type? 

Your blood type is inherited from your parents—and you can’t change it anymore than you can change your eye color. 

Each parent contributes one of their two A, B, or O alleles (a form of a gene) to a baby’s blood type. The O allele is considered recessive, which means it’s not always expressed. So if a woman with OO alleles has a baby with a man who has BB alleles, the baby will have a B blood type. 

Can a baby ever have a blood type different from its parents? “It’s certainly possible,” says Deva Sharma, MD, MS, a hematologist-oncologist at Vanderbilt University Medical Center in Nashville, Tenn. “For example, an AO mother will have blood type A, and a BO father will have blood type B. However, there is a 25% chance they could have a baby with the blood type O (with inheritance of the OO alleles), and a 25% chance they could have a baby with an AB blood type (with inheritance of the A allele from the mother and the B allele from the father).”

What other combinations can occur? Emory University School of Medicine put together this chart:

Parent #1’s allelesParent #2’s allelesBaby’s Blood Type
AA or AO (Type A)AA or AO (Type A)Type A or O 
AA or AO (Type A)BB or BO (Type B)Type A, B, AB, or O 
AA or AO (Type A)AB (Type AB)Type A, B, or AB
AA or AO (Type A)OO (Type O)Type A or O
BB or BO (Type B)BB or BO (Type B)Type B or O
BB or BO (Type B)AB (Type AB)Type B, A or AB
BB or BO (Type B)OO (Type O)Type B or O
AB (Type AB)AB (Type AB)Type A, B, or AB
AB (Type AB)OO (Type O)Type A or B
OO (Type O)OO (Type O)Type O

Your Rh factor is also inherited, and like your blood type, you inherit one of two Rh alleles from each parent. So a baby receiving an Rh positive allele from each parent will be Rh positive, and one receiving a negative Rh allele from each parent will be Rh negative. If you have one positive and one negative Rh allele (making you Rh positive, as the Rh negative allele won’t be dominant), you could pass either one down to your child. Whether or not your baby will be Rh positive or negative will depend on what is also passed down by the other parent.

How do I find out my blood type?

There are three ways you can find out your blood type.

  1. Your healthcare provider can order a blood type test.
  2. You can donate blood. A typing test will be done and the results sent to you.
  3. You can purchase an at-home blood typing test. These tests usually involve pricking your finger and putting a drop of blood on a chemically treated card that looks for antigens and the Rh factor. You then match what you see on the card to a provided guide. Other tests may involve a saliva sample. 

These tests aren’t foolproof, though. “There are certain scenarios in which we find discrepancies in our blood typing,” says Dr. Sharma. “This can occur in a person with a blood cancer, for example, or in someone who has had a recent blood transfusion or stem cell transplant.”

What blood types are compatible for transfusions?

While it seems counterintuitive, knowing your blood type isn’t absolutely critical. “Surprisingly, many people go through their entire lives without knowing their blood type and it causes them no harm,” says Jerry E. Squires, MD, Ph.D., a professor at the Medical University of South Carolina in Charleston. “Why? Because no hospital is going to transfuse a patient without first doing tests to determine the patient’s blood type. And, no, a hospital will not take a patient’s word for their blood type. I know that I am blood group A, but if I need blood, tests are going to be done first to make sure of my type and that safe red blood cell units are selected for my transfusion.” 

Getting transfused with a type of blood that isn’t compatible with yours can be deadly. That’s because antibodies in the foreign blood can trigger an immune response to attack against it, causing a cascade of problems. What blood types are compatible and which ones aren’t? According to Memorial Blood Centers, safe combinations include:

Blood typeCan donate blood toCan receive blood from
A+A+, AB+A+, A-, O+, O-
A-A-, A+, AB-, AB+A-, O-
B+B+, AB+B+, B-, O+, O-
B-B-, B+, AB-, AB+B-, O-
AB+AB+ AB+, AB-, A+, A-, B+, B-, O+, O-
AB-AB-, AB+AB-, A-, B-, O-
O+O+, A+, B+, AB+O+, O-
O-O-, O+, A+, A-, B+, B-, AB+, AB-

What happens if you’re in an emergency with no time for a blood type test? You’ll receive O- blood. Without any antigens or Rh D factor, O- blood is compatible with all other blood types. For that reason, people with O- blood are referred at as “universal donors.” 

According to the American Red Cross, every two seconds someone in this country needs a blood transfusion. That makes blood donations particularly critical. “If you’re healthy, please be a blood donor,” Dr. Squires urges. “There is no substitute for blood, and if people don’t donate we will run out. That would mean no surgeries, no transplants, and no treatment for traumas.”

What does my blood type say about my health?

Can your blood type make you prone to certain diseases? While some experts say any possible effect blood type plays on health is insignificant at best, others say there’s a valid connection. 

“The ABO antigens that make up our blood type are not only expressed on the surface of red blood cells, but they are also present in other human tissues as well,” says Dr. Sharma. “This provides the basis for ABO blood type to have clinical significance for various health outcomes outside the blood system.”

What might some of those health outcomes be? According to Northwestern Medicine, studies show that:

  • People with type O blood have the lowest risk of heart disease while people with B and AB have the highest.
  • People with A and AB blood have the highest rates of stomach cancer.
  • People with type A blood can have a harder time than others managing stressbecause they often produce more of the stress hormone cortisol.

But when it comes to blood type and COVID-19 patients—the disease of the moment—there’s good news. According to a recent study from Harvard Medical School researchers published in the journal Annals of Hematology, blood type has no effect on how sick one becomes with coronavirus (despite initial claims that it might).

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