Blood Typing Systems: ABO, Rh, and the 26 ISBT Blood Group Systems

Beyond ABO: The Full Complexity of Blood Typing

Most people know their blood type as something like “A positive” or “O negative” — a two-part label combining their ABO type and Rh status. But those two systems are just the beginning. Human red blood cells carry hundreds of antigens across 26 distinct blood group systems recognized by the International Society of Blood Transfusion (ISBT). Understanding these systems matters for complex transfusion cases, organ transplants, and managing patients with rare antibodies.

The ABO System: The Foundation of Blood Typing

Karl Landsteiner discovered the ABO system in 1901, identifying three initial blood groups — A, B, and O. A fourth group, AB, was identified the following year. Before this discovery, attempts to transfuse blood between humans (or between animals and humans) frequently caused shock, jaundice, and a blood disorder called hemoglobinuria. Landsteiner’s work provided the scientific basis for safe transfusion.

The ISBT assigns each blood group system a number and letter symbol. ABO is system 001. The system is built on three alleles (A, B, and O) that produce four phenotypes — with A and B being codominant and O recessive. Over a century later, ABO compatibility remains the first and most critical check before any transfusion. For full detail on how ABO typing works, see our blood type facts article.

The Rh System: The Second Most Important

Discovered in 1940 by Landsteiner (again) and Alexander Wiener, the Rh system is ISBT system 004 and is “the most important of the other commonly utilized blood grouping systems.” Its significance comes from two things: how common Rh-positive blood is (about 85% of people) and how dangerous Rh incompatibility can be during pregnancy.

The Rh system involves three closely linked genes on chromosome 1 — C, D, and E — each with multiple antigens. The critical antigen is D. If your red blood cells carry the D antigen, you’re Rh positive. The only people who are truly Rh negative are those with the genotype cde/cde — meaning they carry none of the C, D, or E antigens.

Why it matters for pregnancy: An Rh-negative mother carrying an Rh-positive fetus can develop anti-D antibodies that attack the fetal red blood cells in subsequent pregnancies — a condition called hemolytic disease of the newborn. Routine Rh typing during prenatal care and prophylactic Rh immunoglobulin treatment have dramatically reduced the severity of this complication.

The MNSs System (ISBT 002)

The MNSs system was discovered through animal studies — specifically by injecting human red blood cells into rabbits and guinea pigs and observing the immune responses. The system has two loci: M/N and S/s, producing four antigens: M, N, S, and s. Unlike some other blood group antibodies that arise only after exposure, MNSs antibodies can occur naturally without prior transfusion or pregnancy.

While MNSs incompatibilities are less clinically urgent than ABO or Rh, they can contribute to hemolytic disease of the newborn in certain cases.

The Lewis System (ISBT 007)

The Lewis system is biologically unusual. Unlike most blood group antigens, Lewis antigens are not made by red blood cells directly — they’re produced by tissue cells and absorbed onto the red cell surface from plasma. The system has two main antigens: Le(a) and Le(b).

The Lewis system has a key distinction: Lewis antibodies have never been conclusively linked to hemolytic disease of the newborn. This makes Lewis incompatibility less critical to manage in obstetric settings, even when antibodies are detected.

Lewis status can also change temporarily — pregnancy, for instance, can cause Lewis antigens to weaken or disappear from the red cell surface.

The Kell System (ISBT 006)

Named after Mrs. Kellacher, the patient in whom the antibody was first found, the Kell system is clinically significant because anti-Kell antibodies can cause severe hemolytic disease of the newborn and transfusion reactions.

A rare Kell phenotype (K0 or “Kell null”) is associated with chronic granulomatous disease — a condition affecting the immune system’s ability to destroy certain bacteria and fungi. People with the McLeod phenotype (related to Kell) have weakened red blood cell Kell antigens and can develop anti-Kell antibodies if transfused with normal blood.

The Kell antigen is present in roughly 9% of Caucasians and is less common in other populations.

The Duffy System (ISBT 008)

The Duffy system carries two main antigens: Fy(a) and Fy(b). The phenotype Fy(a-b-) — meaning the person carries neither antigen — is notable for two reasons.

First, it’s an ethnicity-linked phenotype: Fy(a-b-) is relatively common in people of West African ancestry but rare in Caucasians or East Asians. Second, and remarkably, it confers natural resistance to Plasmodium vivax malaria. The Duffy antigen acts as an entry point for the P. vivax parasite into red blood cells. People without it are protected from this form of malaria — which likely explains why the phenotype became prevalent in malaria-endemic regions of West Africa.

For transfusion medicine, Duffy-negative blood can be difficult to source outside of communities with significant African ancestry. This creates real supply challenges for Duffy-negative patients, especially outside regions with large West African diaspora populations.

The Kidd System (ISBT 009)

The Kidd system has a single locus with two major antigens: Jk(a) and Jk(b). Kidd antibodies are clinically important because they’re notoriously difficult to detect — they can appear weak on initial testing but cause severe delayed hemolytic transfusion reactions days after a transfusion. For patients who receive frequent transfusions, Kidd typing is often extended to prevent sensitization.

The Jk(a-b-) null phenotype (absence of both antigens) is found more commonly in Pacific Islander and Asian populations, and is classified as a rare blood type. These individuals are at risk of forming strong anti-Jk3 antibodies if transfused with normal blood.

ISBT Classification: All 26 Blood Group Systems

The ISBT maintains an official list of blood group systems, each with a unique number, symbol, and set of defined antigens. The major systems include:

ISBT #	System Name	Symbol	Key Antigens
001	ABO	ABO	A, B, A,B, A1
002	MNS	MNS	M, N, S, s, U
003	P1PK	P1PK	P1, Pk
004	Rh	RH	D, C, E, c, e
005	Lutheran	LU	Lu(a), Lu(b)
006	Kell	KEL	K, k, Kp(a), Kp(b)
007	Lewis	LE	Le(a), Le(b)
008	Duffy	FY	Fy(a), Fy(b)
009	Kidd	JK	Jk(a), Jk(b)
010	Diego	DI	Di(a), Di(b)
011	Yt	YT	Yt(a), Yt(b)
012	Xg	XG	Xg(a)
013	Scianna	SC	Sc1, Sc2
014	Dombrock	DO	Do(a), Do(b)
015	Colton	CO	Co(a), Co(b)
016	Landsteiner-Wiener	LW	LW(a), LW(b)
017	Chido/Rodgers	CH/RG	Ch1-Ch6, Rg1-Rg2
018	H	H	H
019	Kx	XK	Kx
020	Gerbich	GE	Ge2, Ge3, Ge4
021	Cromer	CROM	Cr(a) and 11 others
022	Knops	KN	Kn(a), Kn(b)
023	Indian	IN	In(a), In(b)
024	Ok	OK	Ok(a)
025	Raph	RAPH	MER2
026	JMH	JMH	JMH

In addition to these 26 systems, the ISBT recognizes six antigen collections — groups of antigens with serological, biochemical, or genetic connections that don’t yet meet all the criteria for a formal blood group system.

Why This Complexity Matters Clinically

For routine blood transfusions, matching ABO and Rh is usually sufficient. But certain patient populations require extended typing across multiple systems:

• Sickle cell and thalassemia patients who receive frequent transfusions gradually develop antibodies to non-ABO antigens. Extended matching (often including Kell, Duffy, and Kidd) significantly reduces alloimmunization rates.
• Organ transplant recipients may need HLA-matching that intersects with blood group antigen compatibility.
• Patients with rare blood types may be negative for high-frequency antigens present in most donors, making finding compatible blood extremely difficult.

Understanding these systems is central to the science of rare blood types and why finding compatible blood for some patients requires international registries and frozen inventories maintained across continents.