Blood compatibility is a matter of life and death. What blood type a patient can receive is a decision that has to made quickly and accurately every time. I’ll walk you through 3 simple steps to make the right call every time when matching blood types and provide quick reference charts of blood compatibilities.
Understanding Blood Type Matching
Red blood cell transfusions require matching the donor’s blood type to the recipient’s blood type. Blood types are determined by the presence or absence of antigens on the surface of red blood cells.
There are 4 possible combinations of antigens on a patients red blood cells.
- Antigen A only
- Antigen B only
- Both Antigen A and B
- Neither Antigen A nor B
An antigen is a substance that triggers an immune response.
The ABO system is used to categorize blood types based on which of the 4 combinations of antigens are present on a patient’s red blood cells.
The Rh System is used to further specify the ABO blood type as either positive or negative for the presence of the Rh protein on the surface of the red blood cell.
A patient who has the Rh protein is protein is Rh positive (Rh+).
A patient without the Rh protein is Rh negative (Rh-).
Together, the ABO and Rh system makes up a person’s blood type.
Blood Type O: has no surface antigens
Blood Type A: has surface antigen A
Blood Type B: has surface antigen B
Blood Type AB: has both antigen A and antigen B
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The Immune System Produces Antibodies to Foreign Antigens
Blood Type O
The immune system of the receiver will produce antibodies against whatever antigens are absent on the receivers red blood cells.
A patient with blood type O has no antigens and so will produce antibodies against antigens A and B if exposed.
This means that if a patient with blood type O is exposed to A or B antigens (blood type A, B or AB) the immune system will identify it as foreign and produce antibodies against it to destroy it.
A patient with type O blood can only receive type O blood.
Blood Type A
A patient with blood type A has A antigens. B antigens are absent so the immune system will produce antibodies against B antigens if exposed.
This means if a person with type A is exposed to type B blood, the body will attack those B antigens resulting in agglutination (clumping of the cells). The body will identify these clumped red blood cells as foreign substances that needs to be eliminated.
Since O has no antigens, it will not induce an immune response when administered to a patient with A antigens.
Blood Type B
Likewise, a patient blood type B has B antigens. It does not have A antigens so it will produce antibodies against A antigens if exposed.
This means if a person with type B blood is exposed to any blood with A antigens (A or AB) the immune system will identify it as foreign to the receiver, triggering an immune response, agglutination and elimination.
Since O has no antigens, it will not induce an immune response when administered to a patient with B antigens.
Blood Type AB
A patient with blood type AB has both A antigens and B antigens. Therefore it will not produce antibodies against either A or B.
This means that if a patient with AB blood is exposed to blood that contains A antigens or B antigens (A, B or AB), there would be no immune response.
Since O has no antigens, it will also not induce an immune response when administered to patients with both A and B antigens.
This is why AB patients are referred to as universal receivers: they can receive blood from all the blood groups.
We will organize all of this into charts of blood compatibilities for quick recall.
How Rh System Turns 4 ABO Groups into 8 Blood Types
In addition to the ABO type matching, blood must be match for Rh factor before transfusion.
Rh is antigen that is either present (+) or absent ( – ) on the surface of the red blood cell. A blood donor or receiver is either Rh positive or Rh negative. The Rh status is the +/- designation included in a person’ blood type e.g. A+, B-.
If a patient is Rh antigen negative they will develop antibodies if exposed to Rh positive blood. This will result in a transfusion reaction.
An Rh+ patient can receive Rh+ or Rh- blood.
An Rh- patient can only receive Rh- blood.
Rh Factor and Pregnancy
In pregnancy, Rh status is crucial. Mother and child may have differing Rh status.
If mother is Rh- and the fetus is Rh+, mom’s immune system will identify the baby as foreign and produce antibodies that will cross the placenta and attack the baby’s red blood cells.
This can cause fatal hemolysis for the current baby and even future Rh+ babies.
Fortunately, mom can be given RhoGAM [Rho(D)] which will prevent the mother from developing antibodies against baby’s red blood cells.
8 Blood Types
With the combinations of 4 ABO types and 2 Rh types there are 8 possible blood types.
Donating versus Receiving Blood
Blood donation and receiving blood are not reciprocal. A person may be able to donate blood to a blood group but will not be able to receive it from that same blood group.
A donor with blood type O can donate to blood type A. A type A donor could not donate to blood type O. This is because donors induce antibodies, receivers produce antibodies.
A blood type O donor has no antigens and so it will not induce the receiver to produce any antibodies.
A blood type O receiver has no antigens and so will produce antibodies against any donor antigens it is exposed to.
From a healthcare provider’s perspective, it is more useful to think of compatibility from the perspective of the recipient. The predominant scenario will be that you have a patient in need a transfusion and you need to know what blood type will be compatible.
How to Divide 8 Blood Group into 2 Categories to Determine Compatibility
A simple way to remember compatibility among the 8 different blood groups is to consider them in 2 groups based on ABO letter matching between donor and recipient. Let’s consider compatibility from the perspective of a patient needing to be matched with a donor.
The 2 groups are:
- Single Letter Receiver: A, B or O
- AB Receiver
Each of these group will then be matched based on Rh status.
If the donor is Rh-, the recipient must also be Rh-. If the donor is Rh+ then the Rh status of the recipient does not matter.
Refer to this chart of blood compatibilities often and it will soon become second nature.
Blood Compatibility for Single Letter Receivers
When your patient is any of the single letter blood groups (A, B, or O) the donor must be an exact letter match or group O.
Once we’ve matched blood based on the ABO letters we then consider the Rh status.
If the receiver is Rh+, the donor blood can either Rh+ or Rh-. If the receiver is Rh- donor blood must be Rh-.
If you learn by observing patterns, you will notice that the chart of blood compatibilities above forms a series of triangles. This can be a helpful way to derive compatibilities for blood type matching.
Blood Compatibility for AB Receivers
When a patient is AB blood type, the donor blood must be atleast a partial letter match or O. The donor can therefore be of any of the the 8 blood groups. Once we have the ABO types matched we can then consider the Rh status.
If the receiver is Rh+, the donor blood can either Rh+ or Rh-. If the receiver is Rh- donor blood must be Rh-.
You can see that a patient with AB+ blood can receive any and all donor types. AB+ patients referred to as universal receivers.
AB- patient also can receive from all the donor types but they are limited to the Rh- groups.
There is a clear triangular pattern to be observed here, much like the table for the single letter group.
I structured all these charts of blood compatibilities in the same way so that they are easy to replicate from memory.
3 Steps to Matching Blood Types
With these 3 simple steps you can derive the compatible blood types for any patient. Let’s walk through some examples below.
Blood Compatibility: Example Questions
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The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.
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