Blood Groups and Compatibilities

    • There are many blood groups in the human population including ABO, Rh, Kidd, Kell, Duffy, MNS and Lewis. The most important of these are ABO and RhD.  Transfusion with ABO incompatible blood can lead to severe and potentially fatal transfusion reactions.  RhD is highly immunogenic and can lead to red cell haemolysis in certain settings. 

      ABO antigens and antibodies

      The ABO blood group is the most important of all the blood group systems.  There are four different ABO blood groups (see Table1), determined by whether or not an individual's red cells carry the A antigen, the B antigen, both A and B antigens or neither.

      Normal healthy individuals, from early in childhood, make red cell antibodies against A or B antigens that are not expressed on their own cells.  These naturally occurring antibodies are mainly IgM immunoglobulins.  They attack and rapidly destroy red cells carrying the corresponding antigen.  For example, anti-A attacks red cells of Group A or AB.  Anti-B attacks red cells of Group B or AB.

       Name of Blood Group Antigens present on
      the red cell surface

      ABO antibodies present
      in the plasma

       Type O

       nil  anti-A and anti-B
      Type A  A antigen  anti-B
      Type B  B antigen  anti-A
       Type AB  A and B antigens  nil

      If ABO incompatible red cells are transfused, red cell haemolysis can occur.  For example if group A red cells are infused into a recipient who is group O, the recipient's anti-A antibodies bind to the transfused cells.  An ABO incompatible transfusion reaction may result in overwhelming haemostatic and complement activation, resulting in shock, renal failure & death (for more information please click here). 

      Rhesus D (RhD) antigen

      There are more than 40 different kinds of Rh antigens.  The most significant Rh antigen is RhD.  When RhD is present on the red cell surface, the red cells are called RhD positive.  Approximately 80% of the Australian population are RhD positive.  The remaining 20% of the population that lack the RhD antigen are called RhD negative.

      Antibodies to RhD develop only after an individual is exposed to RhD antigens via transfusion, pregnancy or organ transplantation.  Anti RhD (or anti-D) antibodies destroy RhD positive red cells and can lead to haemolytic transfusion reactions.  This is of particular importance in pregnancy where anti-D antibodies can cross the placenta from mother to unborn child and lead to haemolytic disease of the newborn.

      As a general rule, RhD negative individuals should not be transfused with RhD positive red cells, especially RhD negative girls and women of childbearing age. If transfusion of an RhD positive product to RhD negative recipient is unavoidable a haematologist should be consulted and administration of anti-D immunoglobulin considered.

      Are they compatible?

      When a transfusion is given, it is preferable for patients to receive blood and plasma of the same ABO and RhD group.  However if the required blood type is unavailable, a patient may be given a product of an alternative but compatible group as shown below. 

      Blood Compatibility

       Patient Type

      Compatible Red Cell Types

       Compatible Plasma Types
      (FFP & Cryoprecipitate)


      A, O

      A, AB


      B, O

      B, AB



      O, A, B, AB


      AB, A, B, O


       RhD Positive

       RhD Positive
       RhD Negative

      RhD Positive
       RhD Negative 

       RhD Negative

       RhD Negative

      RhD Positive
       RhD Negative 

      Note that Group O RhD negative (O negative) red cells have neither ABO nor RhD antigens on their surface.  O RhD negative red cells are issued in emergency situations where life saving transfusion is required prior to completion of a crossmatch.  Both RCH and RWH blood banks maintain a reserve of 5 emergency O RhD Negative red cells.  (Click here for further information on emergency blood release).  Group O is often referred to as the universal red cell donor.

      Group AB individuals have neither anti-A nor anti-B antibodies in their plasma. Group AB plasma can therefore be given to patients of any ABO blood group and is often referred to as the universal plasma donor.

      Avoiding ABO incompatible transfusions

      Most ABO incompatible transfusions occur as a result of improper patient identification at the time of collection of the pre-transfusion sample or administration of the blood product.  The pre-transfusion check is carried out at the bedside by 2 members of clinical staff to ensure the right blood is transfused to the right patient.  Positive patient identification prior to blood sample collection and labelling the specimen tube at the bedside is critical for accurate sample collection.

      Other blood cell antigen-antibody systems

      There are many other antigen systems expressed on red cells, white cells and platelets.  Transfusion can cause antibodies to develop in the recipient.  Some of these antibodies can cause transfusion reactions or damage the foetus.  The purpose of pretransfusion testing (or crossmatching) is to detect potentially harmful antibodies in a patient before transfusion and where possible select red cell units that will not react with them.