Find out what a blood group …

The ABO blood group is the most important of all the blood group systems.  There are four different ABO blood groups - A, B, AB & O 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. 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.