ABO Blood Group

Excerpt

There are four common blood groups in the ABO system: O, A, B, and AB. The blood groups are defined by the presence of specific carbohydrate sugars on the surface of red blood cells, N-acetylgalactosamine for the A antigen, and D-galactose for the B antigen. Both of these sugars are built upon the H antigen—if the H antigen is left unmodified, the resulting blood group is O because neither the A nor the B antigen can attach to the red blood cells.

Individuals will naturally develop antibodies against the ABO antigens they do not have. For example, individuals with blood group A will have anti-B antibodies, and individuals with blood group O will have both anti-A and anti-B. Before a blood transfusion takes place, routine serological testing checks the compatibility of the ABO (and Rh) blood groups. An ABO incompatible blood transfusion can be fatal, due to the highly immunogenic nature of the A and B antigens, and the corresponding strongly hemolytic antibodies (1).

Compared to other blood groups, individuals with blood group O may have a lower risk of pancreatic cancer and thromboembolic disease (2, 3). In addition, in certain African populations, individuals with the blood group O may be protected from life-threatening malaria (4). However, this blood group is not more common in some regions where malaria is endemic. This might be because individuals with blood group O are at higher risk of cholera and severe diarrhea due to Vibrio cholerae 01, with individuals with the AB blood group being the most protected (5, 6).

Over 80 ABO alleles have been reported. The common alleles include A1, A2, B1, O1, O1v, and O2 (7). Whereas the A and B alleles each encode a specific glycosyl-transferring enzyme, the O allele appears to have no function. A single-base deletion in the O allele means that individuals with blood group O do not produce either the A or B antigens. Blood type frequencies vary in different racial/ethnic groups. In the US, in Caucasians, the ratio of blood group O, A, B, and AB is 45%, 40%, 11%, and 4% respectively. In Hispanics, the distribution is 57%, 31%, 10%, and 3%; and in Blacks, 50%, 26%, 20%, and 4% (8).