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Blood Types

Overview

All blood does the same thing, but not all blood is the same. Blood types classify blood so healthcare providers can determine whether one person’s blood is compatible with another’s. Blood transfusions and organ transplants depend on donors and recipients having compatible blood types. Blood types include A, B, AB and O.

While everyone’s blood contains the same basic parts, there are eight different blood types. The type you have depends on genes you inherit from your biological parents.

What makes your blood different from someone else’s is your unique combination of protein molecules, called antigens and antibodies. Antigens live on the surface of your red blood cells. Antibodies are in your plasma. Your blood type is based on the combination of antigens and antibodies in your blood.

Blood basics

Most people have about 4 to 6 liters of blood in their body. Your blood is made up of different kinds of cells that float in plasma, a fluid made up of salts and proteins. The different cell types in your blood include:

Red blood cells, which deliver oxygen to tissues and remove carbon dioxide

White blood cells, which destroy invaders and fight infection

Platelets, which help your blood clot

What are blood types?

A blood type is a classification system that allows healthcare providers to determine whether your blood is compatible or incompatible with someone else’s blood. There are four main blood types: A, B, AB and O. Blood bank specialists determine your blood type based on whether you have antigen A or B on your red blood cells. They also look for a protein called the Rh factor. They classify your blood type as positive (+) if you have this protein and negative (-) if you don’t.

This makes for eight common blood types:

A positive (A+). This is one of the most common blood types, with 35.7% of the U.S. population having this type). If you have this type, you can give blood only to people who are A positive or AB positive.

A negative (A-). If you have this rare blood type (like 6.3% of the U.S. population), you can donate blood to anyone with A or AB blood type.

B positive (B+). Another rare type, it is found in 8.5% of people. It is compatible only with B positive or AB positive blood.

B negative (B-). Someone with this very rare blood type can give blood to anyone with B or AB blood type. Only about 1.5% of Americans are B negative.

AB positive (AB+). If you have this rare blood type (found in 3.4% of Americans), you can receive blood or plasma of any type. You’re known as a universal recipient.

AB negative (AB-). his is the rarest blood type, with only 0.6% of the U.S. population having it. If you have this blood type, you’re called a “universal plasma donor,” because anyone can receive this type of plasma.

O positive (O+). This is one of the most common blood types, found in 37.4% of people. O-positive donors can give blood to anyone with a positive blood type.

O negative (O-). If you have this rare blood type like 6.6% of Americans, you can donate it to anyone with any blood type.

Knowing about blood types allows healthcare providers to safely transfuse donated blood from one person into another during a blood transfusion. Blood types also need to be compatible for organ transplants.

How do blood types get determined?

Blood bank specialists determine blood types based on whether there are particular antigens on your red blood cells. An antigen is a substance that can make your body’s immune system react. Think of an antigen as a marker (like a nametag) that identifies a substance in your body as belonging or not belonging.

Your blood type is compatible with someone else’s if your immune system recognizes the antigens in donated blood as belonging.

What blood types mean

Most people think of A, B, AB and O when they hear the phrase “blood types.” These letters classify blood types based on whether red blood cells have the A antigen or B antigen. This is called the ABO system.

Type A: Red blood cells have the A antigen and B antibody.

Type B: Red blood cells have the B antigen and the A antibody.

Type AB: Red blood cells have both A and B antigens but neither A nor B antibodies.

Type O: Red blood cells have neither A nor B antigens but has both A and B antibodies.

Blood types are either “positive” or “negative,” depending on the absence or presence of the Rh factor’s D antigen, another marker. This is called the Rh system. Being RhD positive is more common than being RhD negative.

Positive (+): Red blood cells have the RhD antigen.

Negative (-): Red blood cells don’t have the RhD antigen.

What are the different blood types?

There are eight blood types included in the ABO and Rh blood group systems: A positive (A+), A negative (A-), B positive (B+), B negative (B-), AB positive (AB+), AB negative (AB-), O positive (O+), O negative (O-).

The most common blood type in the United States is O positive (O+).

The least common blood type in the U.S. is AB negative (AB-).

Rare blood types

Outside of the ABO system, there are over 600 antigens that may attach to red blood cells. There are more than 30 different blood group systems related to these unique antigens.

Other blood group systems include the:

Duffy blood group.

K antigen (or Kell) group.

Lutheran blood group.

Kidd blood group.

These blood groups are rare. A general definition of a rare blood type is one that happens at a rate of 1 per 1,000 people or fewer.

One of the world’s rarest blood types is Rh-null. Fewer than 50 people in the world have this blood type. It’s so rare that it’s sometimes called “golden blood.”

What Is Golden Blood?

Golden blood is the rarest known type of blood in the world. It has no Rh antigens at all, known as Rh-null. It is dubbed "golden blood" because it can be donated to people with almost any Rh blood type, including those with rare types of Rh antigens.

However, if people with golden blood need blood, they can only receive the same type of blood. Experts estimate that only about 50 people are known to have golden blood, which was first detected in Australian aboriginal people.

How are blood types inherited?

Genetics and ABO blood type

Like many of your physical characteristics, including your eye color or dominant hand, your blood type depends on your genetics. Genes you inherit from your biological parents determine your blood type, but the inheritance patterns may not resemble the patterns of other traits or conditions you know. The International Society of Blood Transfusion recognizes 47 blood group systems, and scientists have identified 52 genes responsible for the various blood types in humans. Keep reading to learn how your genes influence some of the most important blood type groups and why it matters.

The ABO gene on chromosome 9 helps determine which blood type you have. You inherit two copies (alleles) of this gene — one from each parent.

This gene has three possible alleles — A, B, and O. A and B are dominant, while O is recessive. That means that if you have an A or B allele with an O allele, you will express the A or B allele. However, if you have both the A and B alleles, you will express both.

This results in four typical outcomes:

Inherited from parent 1 → Inherited from parent 2 ↓	A	B	O
A	A,	AB	A
B	AB	B	B
O	A	B	O

Parent 1	Parent 2	Child’s possible ABO blood type
A	A	A, O
A	B	A, B, AB, O
A	AB	A, B, AB
A	O	A, O
B	B	B, O
B	O	B, O
B	AB	A, B, AB
AB	AB	A, B, AB
AB	O	A, B
O	O	O

Parent 1	Parent 2	Child’s possible Rh blood type
+	+	+ or -
+	-	+ or -
-	-	-

Because of the dominant and recessive patterns of the alleles, it’s possible to have many different blood types within the same family.

For example, if one parent had type A blood with A and O alleles and another parent had type B blood with B and O alleles, they could potentially produce four children with four different blood types.

Which blood types are compatible?

An important part of blood donation is ensuring that someone receiving blood (recipient) has a compatible blood type with someone donating blood (donor). If you receive blood from a donor whose blood cells contain antigens your body doesn’t recognize, your immune system may attack the donated red blood cells. The results could be life-threatening.

Your blood type allows providers to determine which blood types are safe for you to receive. It helps them know which recipients can safely receive blood that you donate.

A positive: You can receive blood that’s A positive, A negative, O positive or O negative.

A negative: You can receive blood that’s A negative or O negative.

B positive: You can receive blood that’s B positive, B negative, O positive or O negative.

B negative: You can receive blood that’s B negative or O negative.

AB positive: You can receive any blood type.

AB negative: You can receive blood that’s AB negative, A negative, B negative or O negative.

O positive: You can receive blood that’s O positive or O negative.

O negative: You can only receive blood that’s O negative.

Blood type	Blood types you can receive	Blood types you can donate to
A+	A+, A-, O+, O-	A+, AB+
A-	A-, O-	A-, A+, AB-, AB+
B+	B+, B-, O+, O-	B+, AB+
B-	B-, O-	B-, B+ AB+, AB-
AB+	All blood types (universal recipient)	AB+
AB-	AB-, A-, B-, O-	AB-, AB+
O+	O+, O-	O+, A+, B+, AB+
O-	O-	All blood types (universal donor)

Note that this assumes that all of the less common Non-ABO blood group antigen systems are also compatible.

Universal donor

Type O negative is often called the universal donor blood type because it can be transfused, or given, to anyone. That’s because type O-negative blood cells don’t have antibodies to A, B, or Rh antigens. Type O-negative blood is usually given in a life-threatening emergency when there’s no time to match blood, or when there’s a limited supply of the ideal blood type. Transfusing it can still carry risks.

If you have type O negative or positive, the blood bank may request your whole blood or just red blood cells. Donors with other types of blood may be commonly asked to donate platelets because they help blood clot, which is beneficial in treating people with blood cancers.

Universal recipient

Blood type AB positive (AB+) is the universal recipient. You can safely receive blood from any other blood type. Your blood recognizes all potential antigens as safe, so your immune system doesn’t launch an attack.

How can you find out your blood type?

You can ask your physician. They might have your blood type on record. Another way, which would be helpful to you and others, is to volunteer to donate blood if you’re eligible. You can also buy at-home blood test kits to determine your blood type.

Blood bank specialists use blood typing to determine what antigens you have on your red blood cells. They mix your blood sample with antibodies that attack antigens A or B. An antibody is a protein in your body that attacks substances that don’t belong, like unfamiliar antigens.
Blood Type Test

If you donate blood, the health care provider will want to make sure that your blood type is the right match for a recipient. Likewise, if you need a transfusion, they will make sure that the blood you receive is compatible with yours.

Labs use a method called blood typing to find out your blood type. The lab technician will run an ABO typing test on your blood by mixing your blood with antibodies against type A and B blood. If your blood reacts and cells stick together, it indicates a match with one of the antibodies, determining your blood type.

The technician can also use a process called back typing to check your blood type. For this test, the worker will mix your blood serum, or liquid part of your blood without cells, with blood that’s either type A or type B. People with type A blood show anti-B antibodies, and vice versa. (Type O has both kinds of antibodies).

Once the lab knows your blood type, they can select a donor blood that’s compatible with yours. Then, they’ll do a crossmatch to make sure it’s the perfect fit.

If you’re a blood donor, check out the report you receive after donating, because it will include your blood type.
Why knowing your blood type matters

Knowing your blood type can be helpful if you ever need to give or receive blood. Certain blood types are incompatible with each other. If you receive an incompatible blood type, your immune system will reject the donor blood, possibly leading to a life threatening situation.

However, hospitals do extensive testing before considering a blood transfusion to ensure a proper blood type match.

It’s especially important for pregnant people to know their blood type. Birthing parents who have blood types that are incompatible with their babies may experience complications, such as hemolytic disease of the fetus and newborn.

Additional Details

Can blood types skip generations?

Yes. Because certain alleles for blood types are recessive, you may inherit a recessive allele that a parent did not express but which they inherited from one of your grandparents. Depending on what other allele you inherit, you may express that allele.

How does blood type affect pregnancy?

If you are pregnant, it's important to identify your Rh blood type so you and your healthcare providers can prevent the consequences of Rh incompatibility. This affects only pregnant people who are Rh negative.

If the pregnant person is Rh negative and the other parent is Rh positive, the fetus may be Rh positive. This is called Rh incompatibility.

This incompatibility will not affect a child born during a first incompatible pregnancy. During birth, however, the blood of the pregnant person and fetus mixes. The Rh negative pregnant person can develop antibodies to the Rh factor.

Those antibodies could harm subsequent fetuses that are Rh positive. The pregnant person's anti-Rh(D) antibodies will identify fetal Rh proteins as foreign and attack them. Fetal red blood cells can swell and tear in response, known as hemolytic disease of the fetus and newborn.

This can lower the fetus's or newborn's red blood cell count and lead to serious consequences, such as brain damage, pregnancy loss, or death of the newborn.

An Rh negative pregnant person who has not developed anti-Rh(D) antibodies should given RhoGAM, or intravenous WinRho, a Rho(D) immune globulin to prevent the development of the antibodies

Can blood types influence your risk of certain health conditions?

A 2020 study found that people with non-type O blood may be at higher risk of thrombotic events, like blood clots or pulmonary embolism. It also found that people with type A may experience less healthy aging than people with type O. However, more research is necessary.

Can your blood type change?

In rare cases, your blood type might change. For example, some people with leukemia experience a temporary blood type change when antigens are no longer expressed on their RBCs while they have cancer.

Another possible situation is a bone marrow transplant, in which the recipient’s blood type may temporarily change to match the donor’s blood type.

One Final Note..

Your blood type — whether a common or rare type — is passed down from your biological parents. It’s a good idea to know your blood type for your own health, especially if you need a transfusion in the future. It’s also important to know what type of blood you carry if you choose to donate blood.

It’s useful to know your blood type. If you’re ever in a position where you need a blood transfusion, determining your blood type will be an essential first step. Knowing which blood types are compatible with yours allows your provider to keep you safe should you ever need blood. The good news is that you don’t need to understand all the microscopic cell markers that come into play to safely donate or receive blood. The blood collection centers of the American Red Cross and local hospital blood bank specialists have controls and safeguards in place to ensure the safety of the blood donation process.

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