ABO and Rh

Blood group systems are necessary to identify suitable products for transfusion.

Their clinical importance also relates to the production of alloantibodies that destroy transfused blood cells or that cross the placenta and give rise to haemolytic disease in the fetus and newborn. This is dependent upon:

  • The frequency with which alloantigens and alloantibodies occur, which may be different among ethnic groups.

  • The functional characteristics of alloantibodies: thermal range, immunoglobulin class, titre, avidity and ability to fix complement.

  • The presence of autoantibodies that are relevant to the diagnosis and management of autoimmune blood disorders and that may interfere with compatibility testing.

ABO blood group

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ABO is the most important of all the blood group systems.

There are four different ABO blood groups, determined by whether an individual's red cells carry the A antigen, the B antigen, both A and B antigens, or neither antigens.

Normal healthy individuals, from early in childhood, make red cell antibodies against A or B antigens that are not expressed in their own cells.

These naturally occurring antibodies are mainly IgM immunoglobulins but can also be IgG. Anti-A reacts with red cells of Group A or AB, and anti-B against red cells of Group B or AB. They bind to red cells carrying the corresponding antigen and trigger an immune response, causing the red cells to haemolyse.

If ABO incompatible red cells are transfused, immediate red cell haemolysis can occur.

For example, if Group A red cells are transfused into a Group O recipient, the recipient’s anti-A antibodies bind to the transfused red cells.

An ABO incompatible transfusion reaction is an acute haemolytic transfusion reaction. This reaction can lead to overwhelming haemostatic and complement activation and result in shock and renal failure. Stopping the transfusion, following the steps for managing a suspected transfusion reaction and immediate volume resuscitation is imperative.

Rh blood group

At present, the Rh system comprises 55 antigens, however new antigens continue to be discovered. The Rh antigens are located on the red cell membrane protein. The D antigen is the most immunogenic and important Rh antigen, followed by c and E. Routine Rh typing of donors and patients only test for the presence/absence of the D antigen.

The presence of D antigen on red cells confers D positivity; while people who lack D antigen are D negative.

Exposure of D negative individuals to even small amounts of D positive cells, by either transfusion or pregnancy, can result in the production of anti-D antibodies which are mainly IgG type.

All Rh alloantibodies should be considered potentially capable of causing (2,3,4):

1. Fung MK, Eder AF, Spitalnik SL & Westhoff CM, 2017, AABB Technical Manual 19th Edition
2. International Society of Blood Transfusion (ISBT). Red cell immunogenetics and blood group terminology. [cited 2020 July 31]. Available from:  https://www.isbtweb.org/fileadmin/user_upload/Table_of_blood_group_antig...
3. Reid ME, Lomas-Francis C, Olsson ML, 2012, The Blood Group Antigen Facts Book 3rd Edition.