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Figure 16-13 ABO blood group antigens. A, Blood group antigens are carbohydrate structures added onto cell surface proteins by the action of glycosyltransferases. Most people inherit a gene that encodes L-fucosyltransferase, which produces the H antigen. Inheritance of a gene for W-acetyl-D-galactosaminyl transferase, which generates the A antigen, and the gene that encodes D-galactosyltransferase, which generates the B antigen, varies between people. A person who does not inherit genes for either of these enzymes will be type O; a person who inherits only one of these glycosal transferase genes will be type A or B; and a person who inherits genes for both enzymes will be type AB.

Figure 16-13 ABO blood group antigens. A, Blood group antigens are carbohydrate structures added onto cell surface proteins by the action of glycosyltransferases. Most people inherit a gene that encodes L-fucosyltransferase, which produces the H antigen. Inheritance of a gene for W-acetyl-D-galactosaminyl transferase, which generates the A antigen, and the gene that encodes D-galactosyltransferase, which generates the B antigen, varies between people. A person who does not inherit genes for either of these enzymes will be type O; a person who inherits only one of these glycosal transferase genes will be type A or B; and a person who inherits genes for both enzymes will be type AB.

heterozygotes) form the A antigen by adding terminal ^-acetylgalactosamine to some of their H antigens. Similarly, individuals who express a B allele (BB homozygotes, BO heterozygotes, or AB heterozygotes) form the B antigen by adding terminal galactose to some of their H antigens. AB heterozygotes form both A and B antigens from some of their H antigens. The terminology has been simplified so that OO individuals are said to be blood type O; AA and AO individuals are blood type A; BB and BO individuals are blood type B; and AB individuals are blood type AB. Mutations in the gene encoding the fucosyltrans-ferase that produces the H antigen are rare; people who are homozygous for such a mutation are said to have the Bombay blood group and cannot produce H, A, or B antigens. These individuals make antibodies against H, A, and B antigens and cannot receive type O, A, B, or AB blood.

Individuals who express a particular ABO antigen are tolerant to that antigen, but individuals who do not express that antigen produce natural antibodies that recognize the antigen. Virtually all individuals express the H antigen, and therefore they are tolerant to this antigen and do not produce anti-H antibodies. Individuals who express A or B antigens are tolerant to these molecules and do not produce anti-A or anti-B antibodies, respectively. However, blood group O and A individuals produce anti-B IgM antibodies, and blood group O and B individuals produce anti-A IgM antibodies. On face value, it seems paradoxical that individuals who do not express a blood group antigen make antibodies against it. The likely explanation is that the antibodies are produced against glycolipids of intestinal bacteria that happen to cross-react with the ABO antigens, unless the individual is tolerant to one or more of these.

In clinical transfusion, the choice of blood donors for a particular recipient is based on the expression of blood group antigens and the antibody responses to them. If a patient receives a transfusion of red blood cells from a donor who expresses the antigen not expressed on self red blood cells, a transfusion reaction may result (described before). It follows that AB individuals can tolerate transfusions from all potential donors and are therefore called universal recipients; similarly, O individuals can tolerate transfusions only from O donors but can provide blood to all recipients and are therefore hematopoietic stem cell transplantation 385

called universal donors. In general, differences in minor blood groups lead to red cell lysis only after repeated transfusions trigger a secondary antibody response.

ABO antigens are expressed on many other cell types in addition to blood cells, including endothelial cells. For this reason, ABO typing is critical to avoid hyperacute rejection of certain solid organ allografts, as discussed earlier in the chapter. ABO incompatibility between mother and fetus generally does not cause problems for the fetus because most of the anti-carbohydrate antibodies are IgM and do not cross the placenta.

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