Updated 13 February 2013

ABO and Rhesus Blood Groups

ABO blood groups divide blood into 4 groups: A, B, AB, O. The other main grouping is the Rhesus system, which is either negative or positive.



  • ABO blood groups are the most basic system of blood typing.
  • This system divides blood into four groups – A, B, AB and O.
  • The other main grouping used is the Rhesus (Rh) system, which is either negative or positive.
  • Your blood group is a combination of the ABO system and the Rh system: for example, O negative.
  • When a blood transfusion is required, your ABO and Rh groups are cross-matched with donated blood.
  • In an absolute emergency anyone can receive O negative blood.

What are blood groups?

There are millions of different molecules on our red cells, each with its own function. These molecules were studied and divided into different group systems according to their structure. There are more than 45 known blood groups on red blood cells – not all of them are clinically significant. In blood transfusion the two most important group systems are the ABO-system and the Rhesus system. When scientists mixed different people’s red cells, they noted that the plasma from certain people made some red cells agglutinate or cluster together. This is because if you lack some of these molecules on your red blood cells, you make antibodies against that molecule.


What are ABO blood groups?

Blood types are classified in several ways. The most basic system is called ABO, which divides blood into four groups, or types: A, B, AB and O.


There are two distinct antigens (a type of protein) present on the surface of some people's red blood cells. Each person's blood contains a specific and inherited set of these. In the ABO system, one antigen is labeled "A" and the other "B." If the red blood cell has only A antigen on it, that blood is called "type A". If the red blood cell has B antigen only, that blood is called "type B". If the red blood cell has both A and B antigens, the blood is called "type AB". If the red blood cell has neither antigen, the blood is called "type O".

In a group of 100 people, on average:

  • 45 are type O
  • 40 are type A
  • 0 are type B
  • 5 are type AB.

This varies among different ethnic populations. A small percentage of people have rare blood types, which appear as an unusual and sometimes extensive series of letters in addition to their ABO type. A blood type is considered rare when more than 200 donors have to be screened to find one compatible donor.

Antigens and antibodies

The blood groups A, B, AB or O can be detected by the presence or absence of antigens on the surface of the red blood cells. They can also be determined by antibodies in the blood plasma. Paired with the red blood cell antigens, your plasma also contains a specific set of antibodies. These are proteins that will attack the antigens on the surface of red blood cells of a different blood type. Consequently, successful blood transfusions require the "matching" of donor and recipient blood type.


People have antibodies only against antigens their red blood cells lack. In other words, people with type A blood have A antigens on their red cells and antibodies against type B antigens in their plasma, while those with type B have B antigens on their cells and antibodies against blood group A in their plasma. The following table outlines the differences:

Blood Group

Antigens present

Antibodies present


A Antigen



B Antigen



A and B Antigens

No antibodies


Neither Antigens

Anti-A and Anti-B

The Rhesus system

The second major blood grouping system is the Rhesus (Rh) system. Like the ABO blood types, the Rh factor is an inherited blood protein, or antigen, on red blood cells. People who have it are "Rh positive"; those who don’t are "Rh negative". (The Rh factor was first discovered in the rhesus monkey, which explains the Rh.)


The Rh system is more complex than the ABO system in that there are 35 different possibilities that can be inherited from each parent. These, however, are roughly grouped into positive and negative types. Being Rh positive is more common than being Rh negative: about 85% of people are Rh positive.

The Rh factor is connected to your ABO blood type. For example, your blood may be AB+, which means you have type AB blood and are Rh positive. Or, you might have O- blood, which means that you have type O blood and are Rh negative.

It is particularly important for expectant mothers to know their blood's Rh factor. Occasionally, a baby will inherit an Rh positive blood type from its father while the mother has an Rh negative blood type. The baby's life could be in danger if the mother's antibodies against the baby's Rh positive blood cells attack these cells. If this happens, an exchange transfusion, in which the baby's blood is exchanged for new blood that matches the mother's, may save the baby's life.

A mother with a Rh negative blood type will make antibodies if she is exposed to the blood from an Rh positive baby during childbirth. Therefore, the formation of antibodies can be prevented if the mother is given anti-Rh antibodies to suppress the formation of antibodies in her blood after the birth of a Rh positive child.

The eight blood types

To summarise, the four major ABO blood types or groups - A, B, AB and O - are each further divided into Rh positive or Rh negative types, putting people generally into one of eight blood groups. The eight groups, and their approximate percentage of the population, are as follows:


  • O Positive 37%
  • O Negative 6%
  • A Positive 34%
  • A Negative 6%
  • B Positive 10%
  • B Negative 2%
  • AB Positive 4%
  • AB Negative 1%

Blood type and blood transfusions

When a blood transfusion is necessary, donor and recipient blood must be compatible with regard to major ABO group and Rh type. If not, the recipient's body will react against the incompatible donor blood cells, leading to complications, maybe even death. If samples of two "incompatible" types of blood are mixed, the antibodies of one will cause the red cells of the other to clump together (agglutinate) in the blood vessels, and then make holes in their cell membranes, bursting the cells and releasing the contents (haemolysis), causing a potentially fatal situation. Therefore, it is important that blood types be matched before blood transfusions take place.


People with AB blood type have both A and B antigens on their cells, and no A or B antibodies in their plasma. As they lack any antibodies, they can receive any type of blood and are known as universal recipients. Those with type O blood have neither A nor B antigens on their cells, so their blood cells will not be agglutinated by any recipient's antibodies. They are therefore known as universal donors. On the other hand, they have both A and B antibodies in their plasma and can receive only type O blood.

Patients who are scheduled to have major surgery can make autologous blood donations (donations of their own blood), so they have a perfect match for transfusions.

Normally patients will receive blood with the same Rh and ABO group. In an emergency, a patient with a different blood group may be given O negative blood, because it most likely to be accepted by all blood types. However, there is still a risk involved. In general it is best to mix blood of matching types and Rh factors.

This table lists what kind of blood can be donated and received by each blood group:

Blood type

Can receive

Candonate to

O Positive Blood Type

O Positive

O Positive

O Negative Blood Type

O Negative

O Positive; O Negative; A Positive; A Negative; B Positive; B Negative; AB Positive; AB Negative


A Positive Blood Type

A Positive; A Negative; OPositive; O Negative

A Positive; AB Positive

A Negative Blood Type

A Negative; O Negative

A Positive; AB Positive; A Negative; AB Negative

B Positive Blood Type

B Positive; B Negative; O Positive; O Negative

B Positive; AB Positive; B Negative; AB Negative


B Negative Blood Type

B Negative; O Negative


B Positive; AB Positive; B Negative; AB Negative


AB Positive Blood Type

A Positive; A Negative; B Positive; B Negative; O Positive; O Negative; AB Positive; AB

AB Positive

AB Negative Blood Type

A Negative; B Negative; O Negative; AB Negative

AB Positive; AB Negative

Blood types and parentage

Because the antigens on the red blood cells are controlled by genes, you can determine the possible blood types of children produced by a given pair of parents.

Each person receives an A, a B, or an O gene from each parent. In this system the A and B genes are co-dominant (equally dominant), and the O gene is recessive. This means that a person whose genetic type is either AA or AO will have blood type A; those with genetic type BB or BO will have blood type B, and those with genetic type OO will have blood type O.

Blood tests can also be used when trying to determine paternity, or rather to exclude some men from being the fathers of some children. For instance, if the parents both have blood type O, then the children must all have blood type O. If a child were to have blood type A, B, or AB, then the presumed father cannot be the real father. If the child's blood type was O, then the presumed father could be the real father - but so might millions of other men, so this is certainly not conclusive proof. The table lists possible and impossible situations:

Parents' Blood Types

Possible Children

Impossible Children

A & A (genetically AA/AO & AA/AO)

A, O


A & B (genetically AA/AO & BO/BB)

A, B, AB, O


A & AB (genetically AA/AO & AB)

A, B, AB


A & O (genetically AA/AO & OO)

A, O


B & B (genetically BB/BO & BB/BO)

B, O


B & AB (genetically BB/BO & AB)

A, B, AB


B & O (genetically BB/BO & OO)

B, O



A, B, AB


AB &

A, B


O & O


A, B, AB

It must be kept in mind that there are rare incidences where the protein that anchors the A and B antigens to the red blood cell is absent and the person may possess the genes and can pass them on but their blood group will appear to be O.

In the Rhesus system, positive dominates negative, so if your genetic type is ++ or +-, your blood type will be Rh positive. You will be Rh negative only if your genetic type is --. If both parents have Rh positive blood and +- genes, they could have children who are ++, +-, or --. In other words, their children could be either Rh positive or Rh negative. Children who are Rh negative can have parents who are either Rh positive or Rh negative.

There are in fact more than a dozen complete blood group systems other than the ABO and the Rh systems. This means you can be very precise when looking at inheritance and family trees, and can make paternity testing far more accurate.

Legally, the only acceptable way of determining paternity is by using a system called Human Leukocyte group A antigen typing, or HLA typing, which looks at the whole complement of proteins found on the surface of white blood cells, and on most cells throughout the body. A person's HLA type is like a genetic fingerprint.

Reviewed by Dr Betsie Lombard, MBChB (Pret), Mmed (Haem Path) (Stell)

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