Blood System Portfolio: ABO Group

The ABO blood group system was first discovered in 1901 by Karl Landsteiner. In 1902 Sturli and Von Decastello discovered the AB group. The ABH antigens do not develop until about 6 weeks of fetal life and the concentration of antigens increases until 3-4 years of age and level out. Antibodies to the ABO blood group are naturally occurring, meaning that the body will develop immunity without previous exposure to the other blood type antigens. They are high titer antibodies and will bind an activate complement in vivo. Antibodies are not able to be detected in serum until 3-6 months of age and titers decrease markedly after the age of 65.

When classifying an individuals blood type is it typically referred to by the forward and reverse groupings. For example, if someone is blood type A, they will have naturally occurring antibodies to type B. That is why it is very important when selecting blood products for a patient that every precaution is taken to match the blood types as best as possible. The ABO antibodies are clinically significant and the hemolytic reactions are immediate and hemolytic.

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The ABH genes that encode for the ABO blood group system encode for a glycosyl transferase enzyme that adds a specific monosaccharide to a precursor substance which results in a distinguishable antigenic structure. The B gene encodes for a-3-D-galactosyltransferase which adds the D-galactose sugar resulting in the B antigen on the RBC surface. The A gene encodes for a-3-N-acetylgalactosaminyltransferase which adds the N-acetyl-D-galactosamine sugar resulting in the A antigen. The H gene encodes for the a-2-L-fucosyltransferase which adds the L-fucose sugar resulting in the H antigen.

The ABH antigens are membrane bound glycolipids. All ABH antigens have type 2 precursors that are chains linked by a 1,4 linkage. To have an A antigen, the H antigen MUST be present. Same with the B antigen, but absence of the A and B antigens with the H antigen present is considered type “O”. The ABO genes are located on chromosome #9 and there are 4 major alleles; A1, A2, B and O. A1 is codominant with B. A2 is recessive to A1, but also codominant with B. O is recessive to all other alleles. The H gene is located on chromosome #19 and there are 2 alleles that can be expressed. H encodes for fucosyltransferase, and “h” is an amorph. Individuals that have the phenotype “hh” are considered to have the bombay phenotype and are extremely rare. These individuals will not produce the H antigen, therefore there are no A or B antigens as well. It doesn’t matter if the genes for the A and B antigens are present, without the H antigen, there can be no A or B antigen production.

ABO antibodies are primarily IgM that react at room temperature and sometimes at 37 degrees celsius. The antibodies follow Landsteiners law which states an individual possesses antibodies to ABO antigens that are absent from their own cells. As mentioned above, the forward and reverse grouping should agree. If there is a discrepancy there is further testing that can be done to distinguish the problem. Discrepancies can be found in a previous article written titled “ABO discrepancies”. One such test is called the anti-A1 lectin test. 1-8% of A2 individuals will develop anti-A1, although not clinically significant, it can cause discrepancies. Lectin is a plant seed extract that agglutinates specific human cells. It agglutinates to A1 cells, but not A2 cells. Its possible to use this test to help resolve discrepancies.

Stay tuned for the next blood group system discussed.

-Caleb

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