Blood Groups

If samples of blood from different individuals are mixed together, some combinations result in red cells sticking together as clumps (Figure 9a). This is called agglutination, and occurs when the blood groups are incompatible. It is caused when antigens (or agglutinogens) on the red cell membrane react with specific antibodies (or agglutinins) in the plasma. If the quantity (or titre) of antibodies is sufficiently high, they bind to their antigens on several red cells and glue the cells together, which then rupture (haemolyse). If this occurs following a blood transfusion it can lead to anaemia and other serious complications. The most important blood groups are the ABO system and Rh (Rhesus) groups.

The ABO System

The ABO system consists of four blood groups: A, B, AB and O. The precise group depends on the presence or absence of two antigens, A and B, on the red cells, and their respective antibodies, α and β, in the plasma (Figure 9b). The A and B antigens on red cells are mostly glycolipids that differ in respect of their terminal sugar. The antigens are also found as glycoproteins in other tissues, including salivary glands, pancreas, lungs and testes, and in saliva and semen.

Group A blood contains the A antigen and β antibody, and group B the B antigen and α antibody. Group AB has both A and B antigens, but neither antibody. Group O blood contains neither antigen, but both α and β antibodies. Group A blood cannot therefore be transfused into people of group B, or vice versa, because antibodies in the recipient react with their respective antigens on the donor red cells and cause agglutination (Figure 9c). As people of group AB have neither α nor β antibodies in the plasma, they can be transfused with blood from any group, and are called universal recipients. Group O red cells have neither antigen, and can therefore be transfused into any patient. People of group O are therefore called universal donors. Although group O blood contains both antibodies, this can normally be disregarded as they are diluted during transfusion and are bound and neutralized by free A or B antigens in the recipient’s plasma. If large or repeated transfusions are required, blood of the same group is used.

Inheritance of ABO Blood Groups

The expression of A and B antigens is determined genetically. A and B allelomorphs (alternative gene types) are dominant, and O recessive. Therefore AO (heterozygous) and AA (homozygous) genotypes both have group A phenotypes. An AB genotype produces both antigens, and is thus group AB. The proportion of each blood group varies according to race (Figure 9d), although group O is most common (35–50%). Native Americans are almost exclusively group O.

Rh Groups

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