Acquired B Antigen

An uncommon 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 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, that makes the chain resemble the B antigen (which has a galactose at the end; pretty close, but not exactly the same!). This “not really B” antigen reacts with fresh human anti-B as well as certain monoclonal anti-B testing reagents, 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 maybe treating the red cells with acetic anhydride before retyping them (leads to re-acetylation of the A antigen). See my blog post on Acquired B for more details.

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