ABO and Rh blood types

Author: Kevin Kuschel MD

Last update: May 24th, 2023

Blood type is a classification of blood based on the presence or absence of specific antigens (proteins) on red blood cells (erythrocytes). Determining a person’s blood type is crucial in transfusion medicine, as transfusing blood from one person to another with differing blood types may cause an antigen-antibody reaction in the recipient. The recipient’s immune system doesn’t recognize the foreign blood type’s antigens and treats them as foreign bodies.

Over 30 antigens are present on human erythrocytes, and each can serve as a basis for defining blood classes. However, the most commonly used blood type systems are ABO and Rh.

This article will explain these concepts.

Blood group	Plasma antibodies	RBC antigen
A blood type	Anti-B antibodies	Antigen-A
B blood type	Anti-A antibodies	Antigen-B
AB blood type	None	Antigen-A and Antigen-B
O blood type	Anti-A and Anti-B	None

ABO blood types

The ABO blood system is based on the presence or absence of two specific antigens: Antigen-A and Antigen-B. These antigens are also called agglutinogens, as they can cause agglutination of blood cells.

A person can have both or just one antigen, meaning they can have:

• Blood type A, with only antigen A present on their erythrocytes
• Blood type B, with only antigen B
• Blood type AB, with both antigen A and antigen B

If a person has neither antigen, their blood type is O.

Each blood type has specific antibodies that react with agglutinogens, causing a transfusion reaction when two different blood types are mixed. These antibodies are called anti-agglutinins. The groups have the following anti-agglutinins:

• Blood type A individuals have antibodies against antigen B (anti-B agglutinins)
• Blood type B individuals have anti-A agglutinins
• Blood type O individuals have both anti-A and anti-B agglutinins
• Blood type AB individuals have no anti-agglutinins

This defines the basic concepts of transfusion medicine within the ABO blood system:

Blood type	Can donate to	Can receive from
A	A, AB	A, O
B	B, AB	B, O
AB	AB	Universal recipient: A, B, AB, O
O	Universal donor: A, B, AB, O	O

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Rh blood types

The Rh blood system is defined by the presence or absence of an antigen called Rh or rhesus factor, which was first discovered in Rhesus monkeys.

A person with the Rh antigen is Rh positive (Rh+), while a person without it is Rh negative (Rh-). Like ABO antibodies, there are anti-Rh antibodies. Specifically, Rh- individuals have anti-Rh antibodies, while Rh+ individuals do not.

The Rh system is particularly important during pregnancy. If the mother is Rh- and the fetus is Rh+, this can cause erythroblastosis fetalis or hemolytic disease of the newborn, in which the mother’s immune system attacks the fetus’s erythrocytes. Since the 1970s, this has been prevented with a specific vaccine called RhoGAM, administered during pregnancy to Rh- individuals. This vaccine prevents hemolytic disease of the newborn in cases where the newborn has an Rh+ blood type by stopping the mother’s immune system from producing antibodies that attack the Rh antigen.

The Rh blood system is often combined with the ABO system, resulting in eight blood types:

• AB+ and AB-
• A+ and A-
• B+ and B-
• O+ and O-

This further refines transfusion principles as follows: A patient can receive blood with the same ABO antigens as their own, plus type O. Rh+ individuals can receive Rh+ or Rh- blood, while Rh- individuals must receive Rh- blood.

Blood type	Can donate to	Can receive from
A+	A+, AB+	A+, A-, O+, O-
B+	B+, AB+	B+, B-, O+, O-
AB+	AB+	Universal recipient: A+, A-, B+, B-, AB+, AB-, O+, O-
O+	All positive groups: A+, B+, AB+, O+	O+, O-
A-	A+, A-, AB+, AB-	A-, O-
B-	B+, B-, AB+, AB-	B-, O-
AB-	AB+, AB-	All negative groups: AB-, A-, B-, O-
O-	Universal donor: A+, A-, B+, B-, AB+, AB-, O+, O-	O-

References

• Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., & Kruse, D. H. (2022). Anatomy and Physiology (2nd ed.). OpenStax. https://openstax.org/details/books/anatomy-and-physiology-2e
• Hall, J. E., & Guyton, A. C. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier, Philadelphia PA
• American Red Cross. (n.d.). Blood types. Retrieved April 20, 2023, from https://www.redcrossblood.org/donate-blood/blood-types.html