Blood groups explained: ABO, Rh factor and who can donate to whom
What ABO and Rh blood groups mean, why antigens and antibodies decide transfusion compatibility, and the standard donor and recipient rules for red blood cells.
A person's blood group depends on antigens on their red blood cells and antibodies in their plasma. The ABO system gives four groups (A, B, AB, O) and the Rh system adds positive or negative, making eight common types, and safe transfusion requires matching donor blood to what the recipient's antibodies will accept.
ABO: antigens and antibodies
Blood typing looks for proteins called antigens on red blood cells. In the ABO system, group A red cells carry the A antigen, group B carry the B antigen, group AB carry both, and group O carry neither. The plasma carries the matching opposite antibodies: group A plasma has anti-B, group B plasma has anti-A, group AB plasma has neither, and group O plasma has both anti-A and anti-B.
This pairing is what makes matching matter. If group A blood, which carries anti-B antibodies, received group B red cells, the antibodies would attack the donated cells and cause them to clump together, a reaction called agglutination. That is why a transfusion must be checked before it is given, not assumed from the group label alone.
The Rh factor and the eight common types
The Rh system adds a second label. If the Rh (D) antigen is present on red cells, the person is Rh-positive; if it is absent, they are Rh-negative. Combined with ABO, this gives eight common blood types: A+, A-, B+, B-, AB+, AB-, O+ and O-.
As a general rule, an Rh-positive person can receive Rh-positive or Rh-negative blood, while an Rh-negative person should receive Rh-negative blood, so that they do not form antibodies against the Rh antigen. Blood typing matters especially in pregnancy: if an Rh-negative mother carries an Rh-positive baby, care is taken to prevent her developing antibodies that could harm the baby's red cells.
Who can donate to whom (red blood cells)
For red blood cell transfusion, the standard rules are: group O can receive only group O; group A can receive A and O; group B can receive B and O; and group AB can receive A, B, AB and O. Because group O red cells carry no A or B antigens, type O blood can be given to any ABO group, which is why group O donors are called universal donors. Strictly, O-negative is the universal red cell donor, since it also lacks the Rh antigen, and AB-positive is the universal recipient for red cells.
Two cautions complete the picture. First, these rules describe red blood cells only; plasma and platelet compatibility follow different rules. Second, hospitals still cross-match blood before transfusion, because minor antigens beyond A, B and Rh can also cause reactions. This article gives general educational information only, not personal medical advice.
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Frequently asked questions
What are the four ABO blood groups?
A, B, AB and O, decided by which antigens (A, B, both, or neither) are present on the person's red blood cells.
What does Rh-positive and Rh-negative mean?
Rh-positive means the Rh (D) antigen is present on the red blood cells; Rh-negative means it is absent. Combined with ABO, this gives eight common blood types.
Who is the universal blood donor?
For red blood cell transfusion, O-negative is the universal donor, because its red cells carry no A, B or Rh antigens for the recipient's antibodies to attack.
Who can receive blood from all groups?
AB-positive is the universal recipient for red blood cells, because its plasma carries neither anti-A nor anti-B antibodies. This applies to red cells only, not plasma or platelets.
Can a person with blood group A receive group B blood?
No. Group A plasma contains anti-B antibodies, which would attack group B red cells. Group A recipients can normally receive group A and group O red cells.