If your blood type is...The Science of Blood Compatibility

The Science of Blood Compatibility

The compatibility of blood for transfusion depends on specific markers, called antigens, found on the surface of your red blood cells (RBCs) and the antibodies circulating in your plasma.

1. The ABO System (A, B, AB, O)

This system is determined by the presence or absence of two main antigens: Antigen A and Antigen B.

Blood Type	Antigens on RBCs	Antibodies in Plasma	Why Incompatibility Occurs
Type A	A Antigen	Anti-B Antibody	The Anti-B antibodies will attack Type B or AB blood.
Type B	B Antigen	Anti-A Antibody	The Anti-A antibodies will attack Type A or AB blood.
Type AB	A and B Antigens	None	Since there are no Anti-A or Anti-B antibodies, it will not attack any blood type.
Type O	None	Anti-A and Anti-B Antibodies	The antibodies will attack any blood type containing A or B antigens (A, B, or AB).

2. The Rh System (Positive vs. Negative)

This system is determined by the presence or absence of the RhD Antigen (also called the Rhesus Factor).

• Rh Positive (+): Has the RhD antigen on the RBC surface. Does not produce anti-Rh antibodies.

• Rh Negative (-): Lacks the RhD antigen. Can develop anti-Rh antibodies if exposed to Rh-positive blood (e.g., during a transfusion or pregnancy).

Universal Donor & Universal Recipient

The universal classifications are based on the compatibility rules for Red Blood Cell transfusions:

Universal Donor: O Negative (O-)

• Why? Type O- red blood cells have NO A, B, or RhD antigens.

• Impact: Since there are no antigens to be recognized as "foreign," the recipient's immune system (antibodies) has nothing to attack. This blood can be safely given to people of any blood type in an emergency.

Universal Recipient: AB Positive (AB+)

• Why? Type AB+ blood has ALL A, B, and RhD antigens.

• Impact: Because the recipient's body already recognizes all three antigens, their plasma contains NO corresponding antibodies (Anti-A, Anti-B, or Anti-Rh). Therefore, they can safely receive red blood cells from any blood type without an immune reaction.

Transfusion Compatibility Grid

The image summarizes this by showing who can donate red blood cells to whom (Donor → Recipient):

Donor Blood Type	Can Donate Red Cells To:
O- (Universal Donor)	ALL (O+, O-, A+, A-, B+, B-, AB+, AB-)
O+	O+, A+, B+, AB+
A-	A-, A+, AB-, AB+
A+	A+, AB+
B-	B-, B+, AB-, AB+
B+	B+, AB+
AB-	AB-, AB+
AB+ (Universal Recipient)	AB+ only

Rules For  Plasma Transfusion

The rules for plasma donation and receipt are the reverse of the rules for red blood cell donation because, in plasma transfusions, we are focused on the antibodies in the donor's plasma, not the antigens on the red blood cells.

The Inverse Rules: Plasma Transfusion

Plasma is the liquid component of blood that contains antibodies. The primary goal in a plasma transfusion is to ensure the donor's antibodies do not attack the recipient's red blood cell antigens.

Donor Blood Type	Plasma Antibodies (The Danger)	Can Donate Plasma To:	Why It Works
Type O	Anti-A and Anti-B	O only	O plasma contains strong antibodies. It must only be given to Type O recipients who have no A or B antigens for the antibodies to attack.
Type A	Anti-B	A, AB	The plasma's Anti-B antibodies won't react with the recipient's A antigen.
Type B	Anti-A	B, AB	The plasma's Anti-A antibodies won't react with the recipient's B antigen.
Type AB	None	ALL (O, A, B, AB)	AB plasma is the Universal Plasma Donor because it contains no Anti-A or Anti-B antibodies. It can be given safely to anyone.

Key Takeaways for Plasma:

1. Universal Plasma Donor: AB Plasma. (It has no Anti-A or Anti-B antibodies to cause harm.)

2. Universal Plasma Recipient: O Plasma. (They can only receive O plasma, as all others have dangerous antibodies.)

3. Rh Factor: The Rh factor is generally not as critical in plasma donation as it is in red blood cell donation, though some guidelines recommend matching the Rh factor, especially for women of childbearing age.