Understanding blood groups is essential for performing successful blood transfusions. Red blood cells are covered with a large number of genetically-determined antigens on which the blood group system is based. If a person receives a transfusion of blood which contains new or foreign antigens, the recipient will mount an immune response to the donated blood. This may kill them, and rather often did until it was understood (see history of blood transfusion).
The main blood typing systems are the ABO and the Rhesus system, but there are several other minor blood groups (minor as in uncommon - as any transfusion reaction is potentially serious).
Conventional means of typing blood involve mixing the patient's blood (either the serum or erythrocytes, often both) with known antigens and antibodies to look for clumping of red blood cells. This can be assessed visually either on a flat surface or by measuring the movement of the antibody/antigen complex through a gel.
First discovered by Karl Landsteiner in 1901 with full description by him in 1909. The differences in the antigens are due to polymorphisms in glycosyl transferases. These are enzymes that add modified carbohydrate moieties to the existing protein, which is the H antigen. Type O blood is essentially unmodified, expressing only the H antigen, and therefore is the universal donor type blood.. The very rare hh (Bombay Phenotype) lack all ABO group antigens, including the H antigen, and must never be transfused with any ABO blood so as to prevent an acute haemolytic transfusion reaction. Any apparent O group person should be tested for hh, although hh are universal donors as are blood group O. The highest incidence of hh is in Bombay at 1:10000.
Research is underway into bacterial glycosidases that could cleave the carbohydrate moieties from A or B type blood to produce a O type blood. However, several technical obstacles remain. Firstly, the process must approach 100% conversion as even small amounts of unprocessed A or B antigens may cause a reaction. There is also concern that the enzymes may not entirely specific and could unmask new antigen epitopes on previously inert proteins. Additionally, the red cells will need to withstand the treatment, including purification to remove the bacterial enzymes.
It is unclear why antibodies are present in people who have no prior exposure to blood transfusions. One theory is that bacterial antigens may mimic antigens on the red cells.
Some diseases have an association with blood group, e.g. gastric cancer.
|Group||Genotype||ABO Antigens on red cell||Antibodies||Comments||ABOH Antigens on red cell|
|O||OO||none||anti-A anti-B||universal donor||H|
|A||AA AO||A||anti-B||A H|
|B||BB BO||B||anti-A||B H|
|AB||AB||A B||none||universal recipient||A B H|
|Bombay||hh||none||anti-A anti-B anti-H||universal donor||none|
Named after our distant cousin the monkey, in which this system was first observed.
Rhesus incompatibility is a significant, potentially severe and very preventable problem in later pregnancies where the mother is Rh -ve and the father Rh +ve.