Of the more than 45 known blood groups on red blood cells, two are important with regards to blood transfusions: the ABO-system and the Rhesus system.
The ABO system divides blood into four groups, or types: A, B, AB and O.
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In a group of 100 South Africans, on average:
45 are type O
40 are type A
11 are type B
4 are type AB.
This varies among different ethnic populations. A small percentage of people have rare blood types, which appear as an unusual and sometimes extensive series of letters in addition to their ABO type. A blood type is considered rare when more than 200 donors have to be screened to find one compatible donor.
Antigens and antibodies
The blood groups A, B, AB or O can be detected by the presence or absence of antigens (labeled A and B) on the surface of the red blood cells. Each person's blood contains a specific and inherited set of these antigens. Paired with the red blood cell antigens, your plasma also contains a specific set of antibodies (labeled anti-A and anti-B). These are proteins that will attack the antigens on the surface of red blood cells of a different blood type. Consequently, successful blood transfusions require the "matching" of donor and recipient blood type.
The composition of antigens and antibodies of each ABO blood group is: Blood group A has A antigens and anti-B antibodies. The B-antibodies of a person with A-type blood will attack donor blood with B antigens, namely B-type blood and AB-type blood. It will not react against A-type and O-type donor blood. Blood group B has B antigens and anti-A antibodies. The A-antibodies of a person with B-type blood will attack all donor blood with A antigens, namely A-type and AB-type blood. It will be compatible with B-type and O-type donor blood. Blood group AB has A and B antigens, and no antibodies. A person with AB-type blood has no antibodies to attack any donor blood. Therefore he/she can receive blood from donors with A-type, B-type, AB-type and O-type blood. People with type AB blood are known as universal receivers. Blood group O has no antigens and anti-A and anti-B antibodies. The anti-A and anti-B antibodies of a person with O-type blood will attack all donor blood with either A or B antigens or both and will not be able to receive any blood from donors with Type A, B or AB blood. Only donor blood from Type O donors will be compatible. Blood donors who are of Blood Type O are known as 'universal donors', because their blood can be used for other blood types in an emergency. That is why approximately half of all blood collected is of Group O. There are also slightly more people who are blood type O in South Africa than is the norm in other countries.
Rhesus negative or positive
The second major blood grouping system is the Rhesus (Rh) system. Like the ABO blood types, the Rh factor is an inherited blood protein, or antigen, on red blood cells. People who have it, are "Rh positive"; those who don’t, are "Rh negative". About 85% of people are Rh positive.
The Rh factor is connected to your ABO blood type. For example, your blood may be AB+, which means you have type AB blood and are Rh positive. Or, you might have O- blood, which means that you have type O blood and are Rh negative.
It is particularly important for expectant mothers to know their blood's Rh factor, especially if it is Rh negative. If a Rh negative woman expects the baby of a Rh negative father, the baby has a 100 percent chance of being Rh negative, and all will be well. However, if the baby’s mother is Rh negative and the father Rh positive, chances are either 100 percent or 50 percent (depending on the father’s genes) for the baby to be Rh positive. The baby's life could be in danger if the mother's antibodies against the baby's Rh positive blood cells attack these cells. If this happens, an exchange transfusion, in which the baby's blood is exchanged for new blood that matches the mother's, may save the baby's life.
The mother will produce antibodies if she is exposed to the blood from an Rh positive baby during childbirth or when the placenta tears and some of the baby’s blood enters her bloodstream. Therefore, the formation of antibodies can often be prevented if the mother is given anti-Rh antibodies to suppress the formation of antibodies in her blood after the birth of her first Rh positive child.
Compatibility
When a blood transfusion is necessary, donor and recipient blood must be compatible with regard to major ABO group and Rh type. If not, the antibodies in the recipient's blood will react against the incompatible donor blood cells, causing the red cells of the donor blood to clump together (agglutinate), and then burst (haemolysis). This can be fatal.
Normally patients will receive blood with the same Rh and ABO group. In an emergency, a patient with a different blood group may be given O negative blood, because it most likely to be accepted by all blood types. However, there is still a risk involved. In general it is best to mix blood of matching types and Rh factors.
What kind of blood may be donated and received by each blood group?
With regards to ABO groups, the same principles apply as mentioned above.
With regards to Rh types, Rh positive may only donate to Rh positive, but Rh negative may donate to Rh positive or negative.
The revised table will look like this: Blood group A
A+ (34 percent of population) may receive A+, A-, O+, O-; may donate to A+, AB+;
A- (6 percent) may receive A-, O-; may donate to A+, A-, AB+, AB-.
Blood group B
B+ (10 percent) may receive B+, B-, O+, O-; may donate to B+, B-, AB+, AB-;
B- (2 percent) may receive B-, O-; may donate to B+, B-, AB+, AB-
Blood group AB
AB+ (4 percent) may receive from all (A+, A-, B+, B-, AB+, AB-, O+, O-); may donate to AB+;
AB- (1 percent) may receive form all the Rh negatives (A-, B-, AB-, O-); may donate to AB+ and AB-.
Blood group O
O+ (37 percent) may only receive O+; may donate to all the Rh positives (O+, A+, B+, AB+);
O- (6 percent) may receive O+ and O-; may donate to all (O-,O+, A+,A-, B+,B-, AB+, AB-).
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