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.


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.



ABO Discrepancies


ABO discrepancies can happen for a variety of reasons and it is important to learn to recognize a discrepancy and what to do to correct it.

Clerical and technical errors are by far the most common ABO discrepancy and are usually fairly easy to correct. It is important to record the results of each reaction as you read each tube individually, don’t wait to record the results at the end as this increases the chance of recording an incorrect results. Another common error is writing down the results for the wrong patient, it is important that one patient is run at a time as this gives the Technologist the time to devote to each individual patient.

Failure to add serum or reagent can lead to technical errors and failure of a reaction to occur when one is expected. One of the golden rules of blood bank is to add the reagent antisera and serum BEFORE adding the patients or reagent red cells. Contaminated reagents can lead to erroneous results such as false positives or false negatives. It is important to centrifuge properly as with under centrifugation can alter antibody binding. Over centrifugation can lead to false positives while reading the reaction while there is still a button on the bottom of the tube. ABO antibodies are IgM which react between 4-20 degrees Celsius. Warming the reaction can cause a false negative.

Weak or missing antibodies can occur for many reasons. Newborns don’t develop antibodies until at least 6 months of age. Elderly individuals lose the ability to maintain antibody levels so they may appear weak or missing upon serum testing. Certain patient conditions such as immune deficiencies, chemotherapy, radiation therapy, and bone marrow transplantation explain the discrepancy.

Resolution is through adding two drops of serum in the event that it wasn’t added the first time. Then centrifuge. If still negative then incubate at 4-18 degrees Celsius for 15-30 minutes.

Rouleaux formation can cause unexpected agglutination in all serum tests. Rouleaux is a phenomenon as a result of multiple pathologies such as multiple myeloma, macroglobulinemia, and liver disease. Rouleaux will appear as agglutination macroscopically, but microscopically will appear as stacks of coins. Resolution is through saline replacement.

Weak or missing antigens can be due to the patient having a weak subgroup of A or B, acute leukemia, massive transfusions (Group O), or history of bone marrow transplant. Care should be taken to receive recent transfusion history and any other clinical history. Read the forward typing microscopically to confirm any reactions. Use Anti-A,B and incubate at 4-22 degrees Celsius for at least 15 minutes. Monoclonal antisera can be used that reacts with weak subgroups of A and B.

The acquired B phenomenon can be see in patients who have problems with the colon or infections with gram-negative rods such as E. coli. Bacterial enzymes modify the A antigen to a B antigen and can cause forward typing AB and reverse typing as A. Resolution is to check history for colon infections. Setting up an auto-control can distinguish between a true B and acquired B. The patients own Anti-B will not cause agglutination with the AB cells. Putting the anti-B reagent in a pH 6 solution will acidify it. After re-testing the acquired B antigens will not react with the antisera. Normal B antigens will.

Polyagglutinable cells such as Whartons Jelly, which is found in cord blood will show discrepancies in the forward typing. Whartons Jelly coats the newborns blood so the child may appear type AB. It is important to re-type the newborn after washing 4-5 times to remove the Whartons Jelly.