See also determining ABO Blood type   See also galactosidases   See removing A/B antigens from blood cells

Blood group antigens are either sugars or proteins, and they are attached to various components in the red blood cell membrane. For example, the antigens of the ABO blood group are sugars. In contrast, the antigens of the Rh blood group are proteins.ygotes of IAi heterozgotes.

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The ABO blood group system was the first to be discovered and is the one of greatest importance from the standpoint of blood transfusion. Individuals of blood types A, B and O express A, B and H antigens respectively. These antigens are found on the surface of erythrocytes and platelets as well as on the surface of endothelial and epithelial cells. The major blood product used for transfusion is erythrocytes, which are red blood cells containing hemoglobin. The blood group structures are glycoproteins or glycolipids.

The ABO blood group specificity is determined by the nature and linkage of monosaccharides at the ends of the carbohydrate chains. The carbohydrate chains are attached to a peptide (glycoprotein) or lipid (glycosphingolipid) backbone, which are attached to the cell membrane of the cells. The immunodominant monosaccharide determining type A specificity is a terminal alpha1-3 linked N-acetylgalactosamine (GalNAc), while the corresponding monosaccharide of B type specificity is an alpha1-3 linked galactose (Gal). Type O cells lack either of these monosaccharides at the termini of oligosaccharide chains, which instead are terminated with alpha1-2 linked fuctose (Fuc) residues.

Blood of group A contains antibodies to antigen B. Conversely, blood of group B contains antibodies to antigen A. Blood of group AB has neither antibody and blood group O has both. An individual whose blood contains either (or both) of the anti-A or anti-B antibodies cannot receive a transfusion of blood containing the corresponding incompatible antigen(s). If an individual receives a transfusion of blood of an incompatible group, the blood transfusion recipient’s antibodies coat the red blood cells of the transfused incompatible group and cause the transfused red blood cells to agglutinate. In order to avoid red blood cell agglutination, transfusion reactions, and hemolysis, transfusion blood type is cross matched against the blood type of the transfusion recipient. For example, a blood type A recipient can be safely transfused with type A blood, which contains compatible antigens.

Because type O blood contains no A or B antigens on its RBC membranes, it can be transfused into any recipient with any blood type. People with bloods types A, B, and AB have no antibodies to reject type O. Type O blood is often referred to as the “universal donor” for this reason.

In an attempt to increase the supply of type O blood, methods have been developed for converting certain type A, B and AB glood to type O blood using galactosidases which enzymatically remove the immunodominant monosaccharides specifying the blood group A and B antigens. ygotes.

Gene Organization of teh ABO blood groups:

Mendel always looked at genes with two alternative alleles. Although any dipoloid inividual can carry only two alleles for a gene, there may be more thna two alleles in a population. The ABO blood types in humans invovles three alleles. 

The gene that determines ABO blood types encodes an enzyme that adds sugar molecuels to proteins on teh surface of red blood cells. These sugars are in trn recognition markers for the immune system.

The gene that encodes the enzyme, designated I, has three common alleles: IA, whose product adds galactosamine, IB, whose produce adds galactose, and i, which codes for a protein that does not add a sugar. The gree alleles of the I gene can be cominbed to produce six different genotpyes. An individual heterozygous for the IA and IB alleles produces both forms of the enzyme and exhibits both galactose and galactosamine on red blood cells. Because both alleles are expressed simultaneously in heterozygotes, the IA and IB alleles are condominant. Both IA and IB are dominant over the i allele, becasue both IA and IB alleles lead to sugar addition, whereas the i allele does not. The different combinations of the three alleles produce the following four different phenotypes:

(1) Type A inidivduals add only galactosamine. They are either IAIA homozgotes or IAi heterozyals add only galactose. 

(2) Type B individuls add only galactose. They are either IBIB homozygoes or IBi heterozygotes.

(3) Type AB individuals add both sugars and are IAIB heterozgotes.

(4) Type O individuals add neither sugar and are ii homozygotes. 

The Specific Antigens on A, B and O RBCs

Individuals of blood types A, B and O express A, B and H antigens respectively. These antigens are not only found on the red cells but on the surfaces of all endothelial and most epithelial cells as well.

N-Acetylgalactosamine (GalNAc): is an amino sugar derivative of galactose. It is the terminal carbohydrate forming the antigen of blood group A. It is typically the first monosaccharide that connects serine or threonine in particular forms of protein O-glycosylation.

ABH Antigen Subgroups

The ABH antigens are divided into subgroups, depending on the inner core saccharide chain. As an example, both A, B and H antigens are expressed on type 1 (Galbeta1, 3 GlcNAc), type 2 (Galbeta1,4GlcNAc), type 3 (Galbeta1,3GalNAcalpha) and type 4 (Galbeta1,3GalNAcbeta) chains. (Holgersson (US8,404,456).