Blood Groups

Blood groups are systems of classifying blood according to the different antigens (marker proteins) on the surface of red blood cells (RBCs) and the antibodies in the blood plasma. The antigens affect the ability of the RBCs to provoke an immune response.

The two principal blood grouping systems used are the ABO system and the rhesus system.

ABO Groups

In this system, the presence or absence of two types of antigen (A and B) on the surface of the red blood cells determines a person’s blood group. People with the A antigen (blood group A) have anti-B antibodies; people with the B antigen (blood group B) have anti-A antibodies; those with both antigens (blood group AB) have neither type of antibody; and those with neither antigen (blood group O) have both types of antibody.

Rhesus factors

The rhesus system involves several antigens, the most important of which is called factor D. People with this factor are Rhesus-positive; those without it are Rhesus-negative. The importance of the Rh group relates mainly to pregnancy in Rh-negative women because, if the baby is Rh-positive, the mother may form antibodies against the baby’s blood (rhesus incompatibility).


Blood group typing is essential for safe blood transfusion. The ABO and rhesus groups are used to categorize blood stored in blood banks, so that donor blood that is compatible with that of the patient can be selected before transfusion takes place. Because a person’s blood group is inherited, identification of blood group may be used in paternity testing. Genetic analysis allows identification of the blood of a criminal suspect with virtual certainty.

blood group compatability table

Above: Blood group compatability table. Group A is compatible with A and O, and group B with B and O. Group AB is compatible with all groups, but O is compatible with no other group.

As shown in the table above, 0 Rh(-) is the universal red cell donor blood that can be given to all patients. This is common practice when a patient’s blood group is unknown and in emergency situations especially for women of child bearing age.

Plasma Compatibility

Plasma contains Anti-A and Anti-B antibodies depending upon blood group. Patients should only receive plasma which does not contain an antibody which could attack their own red cells.

Antibodies are important molecules our immune system makes to help protect ourselves against foreign things such as bacteria and viruses. Antibodies can also be formed in response to different blood groups.

Group O people have both Anti-A and Anti-B so group O plasma can ONLY be given to group O patients. If group O plasma were given to a group A patient, the Anti-A will attack the patient’s group A red cells.

Group A plasma contains anti-B. Group A plasma can only be given to patients who are group A or O i.e. only patients who do not have group B red cells.

Group B plasma contains anti-A. Group B plasma can only be given to patients who are group B or O.

Group AB plasma does not contain any Anti-A or Anti-B. AB plasma can be given to patients who are group AB, A, B or O. Group AB donors are called “universal plasma donors” and their plasma can be safely given to any patient.