Blood Types and Matching
Blood tests before the transfusion
Before you get a blood transfusion, tests must be done to make sure a donated blood product closely matches your blood type.
The main blood tests done are :
- Type
- Antibody screen
- Crossmatch
Blood types
Testing and matching are important before someone gets a blood transfusion. If you get a transfusion with a blood type that doesn’t work with yours, your immune system might attack the donated blood. This can cause a serious or even life-threatening transfusion reaction. See Possible risks of blood transfusion for more information on side effects.
Donated blood is always tested to find out what type it is. This is done when it’s taken from the donor and again in the hospital lab. If you need a blood transfusion, a blood sample is taken from you and tested the same way.
All blood has the same parts, but not all blood is the same type. People have different blood types based on antigens. Antigens are substances that trigger the body’s immune response. When typing someone’s blood , two antigens are looked at:
- ABO antigen type. Everyone has either type A, B, AB, or O blood. This means that their blood cells have either antigen A (type A), antigen B (type B), both antigens (type AB), or neither antigen (type O).
- Rh factor. Everyone is either Rh-positive or Rh-negative (you either have Rh or you don’t).
Your ABO antigen + Rh factor = blood type. There are eight different blood types:
- A positive and A negative
- B positive and B negative
- AB positive and AB negative
- O positive and O negative
ABO blood type matching
The A and B antigens decide a person’s ABO blood type (either A, B, AB, or O). In the United States, the most common blood type is O, followed by type A.
Whatever antigen you have on your blood cell is linked with the antibodies in your plasma. Antibodies are proteins in your immune system that watch for and attack foreign substances.
- If you have type A blood with A-antigens, you also make anti-B antibodies. So, you can’t get type B or AB blood, because your anti-B antibodies would attack any donor blood with B-antigens. You can only get type A or O blood.
- If you have type B blood with B-antigens, you also make anti-A antibodies. So, you can’t get type A or AB blood, because your anti-A antibodies would attack any donor blood with A antigens. You can only get type B or O blood.
- If you have type AB blood with A and B antigens, you don’t make anti-A or anti-B antibodies. That is why people with type AB blood can receive transfusions from any blood type. These people are sometimes called universal receivers.
- If you have type O blood with no A or B antigens, you make both anti-A and anti-B antibodies. So, people with type O blood can only get type O blood because their anti-A and anti-B antibodies would attack any donor blood with A or B antigens. But since type O has no antigens, anyone of any blood type can receive type O blood. This is why people with type O blood are sometimes called universal donors.
Rh factor matching
In addition to matching ABO types, blood also needs to be matched by Rh (rhesus) factor . Rh factor is another protein that some people have on their red blood cells.
- If you have Rh-positive blood, you have the Rh antigen on your blood cells. Your body doesn’t make anti-Rh antibodies. So, you can get Rh-positive or Rh-negative red blood cell transfusions.
- If you have Rh-negative blood, you don’t have the Rh antigen on your red blood cells. Your body can make anti-Rh antibodies. So, you should only get Rh-negative red blood cells except in emergencies. An Rh-positive blood transfusion can cause a person with Rh-negative blood to make antibodies against the Rh antigen. This can cause a transfusion reaction.
Other antigens
There are other antigens on red blood cells that can lead to transfusion reactions. These are rare because people don’t make antibodies against them unless they have had transfusions before. These antigens may become a problem in matching blood for a person who has had many transfusions in the past, like some people with cancer.
Plasma, platelets, cryo, and blood type
Blood types are also important for plasma transfusions, but the rules are different from the rules for red blood cells transfusions. For example, people with type AB blood are universal plasma donors. Anyone can receive AB plasma, but someone who is AB can only receive type AB plasma.
For platelet and cryoprecipitate transfusions, matching the blood type of the donor to the recipient is not needed, but many labs still try to match them. If a person gets frequent platelet or cryo transfusions, matching may lower the risk of future transfusion reactions.
Antibody screen and crossmatching
After blood is typed, a test called an antibody screen is done to see if a patient’s plasma has antibodies other than those against A, B, and Rh. If there are extra antibodies, the crossmatching may take longer. This is because some units of donor blood may not fully match the recipient’s, even though they have the same ABO and Rh types.
Before a person can get a red blood cell transfusion, another test called a crossmatch must be done. For a crossmatch, a small amount of donor blood is mixed with the recipient’s blood to see if they react before the blood is transfused. If the two blood samples clump together, they are not a good match and that donor’s blood will not be used for that recipient.
A crossmatch is usually not needed for a platelet or plasma transfusion unless the platelets look like they could have some red blood cells.
Other testing
Other testing might be done depending on the patient. For example, some people with cancer need CMV-negative blood. CMV (cytomegalovirus) is a common infection that most people get at some point, but don't have symptoms. But if someone with a weakened immune system gets CMV-positive blood, it can cause serious problems.
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- References
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.
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Last Revised: June 21, 2023
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