Why is blood group O called “universal donor” group?

“How can a person with blood group A receive blood from someone whose blood group is O? I know that an individual with blood group O does not have the A or B antigen on his/her RBC but has anti-A and anti-B antibodies in plasma”

The question is if anti-A antibodies in donor’s plasma (here the donor happens to be O group) would destroy recipient’s RBC containing A antigens.  This depends upon the amount of blood one is transfusing to the recipient.  If we transfuse about 400 ml of whole blood, donor’s plasma (about 200 ml) would get diluted 10-15 times in recipient’s plasma.  Typically, plasma volume in a normovolemic adult weighing about 70 kg is 3000 ml.  Lysis caused by anti-A antibodies in donor plasma happens to be clinically insignificant.  Antibodies need to be present in significant titers to bring about clinically significant lysis of antigen bearing targets, whereas antigens need be present only in small amounts to activate the immune system.  Thus, individuals with blood group A or B or AB could potentially receive transfusions from a donor whose blood group is O, Rh negative.  This is the reason why the group “O, Rh negative” is often called “universal donor”.

But whether transfusion can proceed depends also on the result of a ‘cross match’ between donor’s RBCs and recipient’s plasma.  This step is essential because there are so many other RBC antigens besides A, B and the D antigen for which donors might harbor antibodies in significant titers.  When large amounts of blood are transfused such as to someone with ongoing blood loss or severe anemia, then, we would expect antibodies transfused from donors’ to build to a significant concentration in recipient’s plasma. Thus, when repeated transfusions are anticipated, it is important to additionally cross match donor’s plasma with recipient’s RBC.  This step is called “minor cross-match”.