In the past, type O Blood was given to virtually anyone except those that were termed 'rare' Blood. Donors of Blood group O were always referred to as 'universal donors'. Today, because of new research and a better understanding of the complex issues regarding immune reaction related to incompatible donor Blood cells, type O Blood is no longer seen suitable. There is no "Universal Donor." With further research, it was seen that even O- donors have some antibodies in their blood that can cause reactions in the recipients. Doctors/nurses now combine or cross-match a small sample of the donor's blood with that of the recipient's to check if they are compatible. The terms universal donor and universal recipient are an over-simplification, because they only consider possible reactions of the recipient's anti-A and anti-B antibodies to transfused red blood cells, and also possible sensitization to RhD antigens (the Rhesus factor or Rhesus D antigens). The possible reactions of anti-A and anti-B antibodies present in the transfused blood to the recipients RBCs are not considered, because a relatively small volume of plasma containing antibodies is transfused. To get a better idea consider the transfusion of O negative blood (still considered universal donor blood) into a recipient of blood group A positive. The possible complications are: - An immune reaction between the recipient's anti-B antibodies and the transfused RBCs is not anticipated. - A relatively small amount of plasma in the transfused blood contains anti-A antibodies, which could react with the A antigens on the surface of the recipients RBCs, but a significant reaction is unlikely because of the dilution factors. Rhesus D sensitisization is not anticipated. - Red blood cell surface antigens other than A, B and Rh D, might cause adverse reactions and sensitization, if they can bind to the corresponding antibodies to generate an immune response. - Transfusions are further complicated because platelets and white blood cellls (WBCs) have their own systems of surface antigens, and sensitization to platelet or WBC antigens can occur as a result of transfusion If an individual is exposed to a blood group antigen that is not recognised as self, the immune system will produce antibodies that can specifically bind to that particular blood group antigen and an immunological memory against that antigen is formed. The individual will have become sensitized to that blood group antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other tissue cells) often leading to destruction of the cells by recruitment of other components of the immune system. It is vital that compatible blood is selected for transfusions and that compatible tissue is selected for organ transplantation. Transfusion reactions involving minor antigens or weak antibodies may lead to minor problems. However, more serious incompatibilities can lead to a more vigorous immune response with massive RBC destruction, low blood pressure and even death. Antibody screening and cross-matching prior to transfusion is necessary as it detects if there are any other blood group incompatibilities between potential donor blood and intended recipients.