The A2B blood group

It was a very long day at work. After 60 hours of continuous duty arrived the most longed for moment in the past two days.....yes exactly,  to go home and sleep like a baby...( the only dream any intern has). While climbing down the stairs to the way out I rechecked my 'TO-DO' list one more time, to make sure am done for the day. Then it came to my notice of the pending job of arranging blood for a patient posted for surgery the early morning next day. To avoid the delay of nursing staff collecting the report, I went to check the report of grouping and typing sample given and order for a unit of blood. The person at the blood bank told me that I was supposed to meet the pathologist on duty before collecting the reports (meeting pathologist is indeed a nightmare lest I would have sent some partially filled blood bank orders or have forgotten to sign an order form both of which are punishable offences an intern does). With great fear, I entered her room only to find there was greater news in store for me. The patient posted for the surgery next day has A2B blood group (OMG!! does such a group really exist??), and it shows cross reactivity. So the patient should be transfused the same group.

I was surprised to learn about the new blood group. I went to the patient and tried to find out from him if he was aware of his blood group. He had checked his blood group before and reported as AB negative. Thus, came the greatest difficult in convincing the patient about the existence of an A2B blood group about which I have learned  no earlier than him (indeed I had some quick review and talks with seniors before meeting him). Luckily someone in his family had the same blood group, and the surgery was done though it had to be postponed for two days before blood group was reconfirmed and arranged. So let me share with you what I have a learned from various sources.

The ABO system of blood grouping comprise of four major groups. A (A1 and A2), B, AB (A1B and A2B) and O. A2B is one of the rare subgroups of ABO comprising less than 1% in the total population and even lesser in the Asian population. But there as many as other 100  subgroups which may complicate transfusion. The difference between the A1 and A2 group is attributed to the genetic variation in the coding regions. The coding region of A2 allele differs from A1 in two ways: 

1. by a single nucleotide substitution that creates a single amino acid change (proline at position 156 is changed to leucine 
2. by a deletion mutation resulting in frameshift, extending the reading frame by 64 nucleotides

These two mutations create an enzyme whose activity is significantly weakened but not completely abrogated.

But there are several other studies suggesting that the difference is attributed to the quantitative difference in A2 antigen in the RBC and some going in favor of qualitative differences as well. Studies are still on regarding this group.

The complications of transfusions mainly arise when it is done under hypothermic conditions as in cardiopulmonary bypassing. Hemolytic reactions are less under normothermic conditions. So this poses a risk when organ transplants are done. Considering autologous transfusions avoid this risk in such cases.

So medicos... please keep in mind of the potential risks that can be encountered while transfusing blood to an A2B person.

Image Courtesy: Environmental Graffiti

References:

1. www.madsci.org/posts/archives/2007-05/1179846995.Me.r.html
2. www.meja.aub.edu.lb
3. www.ncbi.nlm.nih.gov/pubmed/2264401