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It was Karl Landsteiner who developed the ABO blood grouping system in 1901. Up until his discovery many deaths had been caused through blood transfusions, where the donor blood congealed when mixed with the recipient blood. Landsteiner deduced that human beings could be separated into blood groups according to the mode of clotting in the presence of different serum. (http://www.faqs.org/health/bios/26/Karl-Landsteiner.html#ixzz0efZuXypJ 5/02/10). It was described by Landsteiner that the agglutination between the red blood cells and serum was correlated to the existence of antigens on the red blood cells and antibodies in the serum. Agglutination happened when the antibodies in the serum reacted with the antigens on the red blood cells. The antigens were named A and B, and blood either belonged to blood group A or blood group B, dependent on the expression of the antigen. It was found that some blood cells did not agglutinate at all and a third group called O was named. The O group showing no expression for either antigen. The following year the AB blood group was added to the ABO blood group system. It was found that these red blood cells expressed both A and B antigens. (www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=rbcantigen&part=ch05ABO 05/02/10) Where the blood group expressed the A antigen the plasma contained Antibody B, Blood group B expressed the B antigen and the plasma Antibody A. People in the AB grouping had both the A and B antigens but no antibodies in the plasma and those in the O group had no antigens but both A and B antibodies in the plasma.
Figure a. shows the four blood groups and their corresponding antigen / antibody configuration
In blood transfusions, therefore it is of extreme importance that the correct blood group of the patient is established in order to cross match them with the donor blood. If a patient of blood group A was to be given blood from a B group donor the antibodies (anti B) in the plasma of the recipient blood would recognise the B antigen in the donor blood as 'foreign' The antibodies bind with the respective 'foreign' blood cells and agglutination occurs. The agglutinated cells can then congeal and obstruct blood vessels in the body and they can also become unstable and break leaking toxic haemoglobin into the body. (www.nobelprize.org/educational_games/medicine/landsteiner/readmore.html 5/02/2010)
A patient of blood group B given A group blood would react in the same way as the recipient Anti A antibodies would recognise the donor A antigens as foreign. In the case of an AB group patient there are no antibodies present and so blood can be transfused from any group. This group is a universal recipient. A person with O group blood can only receive blood from an O group donor as they have both Anti A and Anti B antibodies, but because they have no antigens they can donate to any group as the cells will not be recognised as 'foreign'. This group is a universal donor.
It is clear that before transfusion takes place the blood cell component of the donor blood, is removed from the plasma component otherwise the antibodies in the donor plasma would recognise the recipient blood as foreign.
When transfusing blood a second factor must be considered along with the ABO blood grouping and it is that of the Rhesus factor. This is also an antigen on the surface of the cell. People who have this antigen are known as Rhesus positive and those who don't Rhesus negative. A person with Rh- blood can donate to a Rh+ positive person as they have no antigen and no antibodies ( Rh antibodies do not occur naturally unlike anti A or B) however Rh+ blood can instigate the production of antibodies in an Rh- person. This does not become a problem on transfusion but can lead to problems later if the patient is exposed to Rh+ blood at a later stage, such as in pregnancy.
By undertaking a series of tests it is possible to determine what antigens are present on red blood cells and therefore establish blood groups. A sample of blood is mixed with different samples of plasma known to contain different antibodies. If plasma which contains anti-A antibodies makes the red cells in the blood clump together, then there are A antigens on the blood cells and the group is A. If plasma containing anti B clumps the blood cells then the blood has B antigens. If clumping occurs with A and B antibodies the group is AB as both antigens are present and if no clumping occurs with anti A or anti B then the group is O. If plasma which contains rhesus antibodies makes the red cells clump together, then rhesus antigens are present on the red blood cells.
Blood group testing is always performed on pregnant women, to establish the Rhesus factor. If the unborn baby is rhesus positive (inherited from a rhesus positive father), and the mother rhesus negative, then the mother's immune system may generate anti-rhesus antibodies which could recognise the baby's blood cells as foreign and attack and destroy them. This is seldom a crisis in a first pregnancy. However, without treatment, this can become a serious problem in subsequent pregnancies as the mother's immune system will be 'sensitised' after the first pregnancy (http://www.patient.co.uk/health/Blood-Test-Blood-Grouping.htm)