Acquired B Antigen

An “uncommon in real life, popular on standardized exams” ABO discrepancy that occurs when red blood cells from blood group A patients come into contact with certain gram-negative bacteria (especially those of colonic origin in cases of colon cancer and gram-negative sepsis). These bacteria may carry an enzyme that removes a part of the A antigen (specifically, the acetyl group from the last sugar on a group A chain, N-acetylgalactosamine), leaving behind a modified sugar, galactosamine. Here’s where things get weird. The modified sugar looks like the most important sugar on a group B chain, so the antigen resembles the B antigen (which has a galactose, not galactosamine at the end). But, let’s be real: It’s not REALLY a B antigen, just a kinda-sorta B antigen. If it WERE a real B antigen, then the patient’s own anti-B (remember, these patients are blood group A) would destroy the patient’s own RBCs.

This “not really B” antigen reacts with certain laboratory reagent anti-B preparations, and can result in an ABO discrepancy. Acquired B is generally no big deal, because, once recognized, it won’t result in a problem during transfusion. However, the blood bank will have to do some tests to confirm Acquired B, such as: Incubating with the patient’s own serum (the group A patient’s own anti-B will not react against acquired B), lowering the pH of the anti-B reaction (acquired B doesn’t react at low pH), and most easily, just using a different reagent anti-B!

For much more, see my blog post on Acquired B.

Updated 03/2024 by Joe Chaffin, MD.

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