Human blood group systems

The term human blood group systems is defined by the International Society of Blood Transfusion as systems in the human species where cell-surface antigens—in particular, those on blood cells—are "controlled at a single gene locus or by two or more very closely linked homologous genes with little or no observable recombination between them", and include the common ABO and Rh antigen systems, as well as many others; 48 human systems are identified as of 2025.

Table of systems and classifications

Antibodies

Following is a comparison of clinically relevant characteristics of antibodies against the main human blood group systems:

	ABO	Rh	Kell	Duffy	Kidd	Lutheran	MNS	Lewis	P	Ii
Most common in immediate hemolytic transfusion reactions	A		Yes	Fy^a	Jk^a
Most common in delayed hemolytic transfusion reactions		E,D,C			Jk^a
Most common in hemolytic disease of the newborn	Yes	D,C	Yes
Commonly produce intravascular hemolysis	Yes				Yes				Yes
Reactive at room temperature	Yes						M,N	Le^a, Le^b	P1
Nearly always clinically insignificant						Yes	M,N	Yes	P1
Naturally occurring	Yes					Yes	M,N	Yes	Yes	Yes
Enhanced by ficain and papain	Yes	Yes			Yes			Yes	P1	Yes
Destroyed by ficain and papain				Fy^a, Fy^b		Yes	Yes
Displaying dosage		Cc, Ee		Yes	Yes		Yes

Compatibility testing

Blood compatibility testing is performed before blood transfusion, including matching of the ABO blood group system and the Rh blood group system, as well as screening for recipient antibodies against other human blood group systems. Blood compatibility testing is also routinely performed on pregnant women and on the cord blood from newborn babies, because incompatibility puts the baby at risk for developing hemolytic disease of the newborn. It is also used before hematopoietic stem cell transplantation, as it may be responsible for some cases of acute graft-versus-host disease.
Other human blood group systems than ABO and Rh have a relatively small risk of complications when blood is mixed. Therefore, in emergencies such as major hemorrhage, the urgency of transfusion can exceed the need for compatibility testing against other blood group systems. Also, blood compatibility testing beyond ABO and Rh is generally limited to antibody detection. Still, in Europe, women who require blood transfusions are often typed for the K and extended Rh antigens to prevent sensitization to these antigens, which could put them at risk for developing hemolytic disease of the newborn during pregnancy.
When needing to give red blood cell transfusion to a patient, the presence of clinically significant antibodies produced by the patient can be detected by mixing patient serum with 2 to 4 "screening" or "control" red blood cells that together display essentially all relevant antigens. If any of these mixes display a reaction, a more extensive antibody panel is warranted.