However, by the time a cross match is done, the level of antibody in the recipient's plasma is too low to cause agglutination, making this type of reaction difficult to prevent. Likewise, during the blood transfusion the level of antibody is too low to cause an acute transfusion reaction. As people with blood group AB have no antibodies, they can receive all blood groups without issues. People with blood group O can only receive RBCs of blood group O, because they carry both antibodies. ... Hence, blood group O can be received by all other groups, and is therefore the universal donor.