When Blood Types Change: ABO Genotype vs. Phenotype

The patient is a composite, built from enough cases to be nobody in particular: an elderly man admitted with a colonic obstruction, febrile, blood cultures eventually growing a gram-negative rod. His chart says A positive. It has said A positive for decades, through two prior admissions and one prior transfusion. The type-and-screen that comes back this time says otherwise. The forward type shows both A and a weak but unmistakable B reactivity. The reverse type, his own serum against reagent cells, still shows anti-B, exactly as it should for a lifelong group A patient. Forward and reverse disagree. Someone on the bench who has seen this before doesn't reach for a rare subgroup workup. They reach for the chart, note the fever and the bowel pathology, and write "acquired B" before the second tube has finished spinning.

He is still, genetically, group A. Nothing about his ABO gene has changed. What's changed is what a bacterial enzyme has done to the surface of his red cells, and that distinction, between what your genes say and what your cells are currently showing, is the whole subject of this post.

Genotype and phenotype, briefly

Your ABO genotype is fixed at conception: the specific alleles you inherited, encoding glycosyltransferase enzymes that add either N-acetylgalactosamine (making A antigen) or galactose (making B antigen) onto a common precursor structure on the red cell surface. Your ABO phenotype is what a lab actually detects: the antigens expressed on your cells, tested by forward typing, and the antibodies circulating in your plasma against the antigens you lack, tested by reverse typing. In the overwhelming majority of people, genotype and phenotype agree completely, which is why we treat "blood type" as a fixed, inherited fact roughly on par with eye color. Most of the time it is. It's just not guaranteed to be, and the exceptions are where transfusion medicine gets interesting.

How the mismatch happens

Acquired B is the cleanest example because the mechanism is fully worked out. Certain gram-negative bacteria, often colonic flora that have gained access through a compromised gut wall from a tumor, obstruction, or infection, carry a deacetylase enzyme. That enzyme strips the acetyl group off the terminal sugar of the A antigen, N-acetylgalactosamine, converting it to galactosamine [1,2]. Galactosamine happens to be structurally close enough to galactose, the terminal sugar that defines the B antigen, that commercial anti-B reagent cross-reacts with it [1]. The cell hasn't started making B antigen. It's wearing a chemically modified version of its own A antigen that a reagent mistakes for B. Pull the source of bacterial exposure, whether by treating the sepsis or resecting the tumor, and the discrepancy resolves on its own, typically within weeks [3]. Genotype never moved. Phenotype took a temporary detour.

Post-transplant chimerism works differently but lands in the same place. A patient who receives an ABO-mismatched hematopoietic stem cell transplant will, over the course of engraftment, gradually stop typing as their native blood type and start typing as their donor's. Eventually the genotype itself has changed, at least in the hematopoietic compartment: the red cells being produced are now genetically the donor's. This one isn't a transient artifact to be explained away. It's a real, durable shift, and it means "blood type" for a post-transplant patient has to be tracked as a moving target rather than looked up once and filed away [4].

A third category lives entirely on the genotype side: weak ABO subgroups, where an inherited variant allele produces a glycosyltransferase with reduced enzymatic activity. The antigen is present, just sparse enough that forward typing looks weak, ambiguous, or occasionally falls out as an apparent O in a person who is not, in fact, group O. No bacteria involved, no transplant involved. Just an inherited enzyme that's technically functional and practically underpowered.

Why it matters beyond the interesting case report

Every one of these scenarios is a reminder that "check the type" and "trust the historical type" are not the same instruction, and blood banks build entire policies around knowing when to prefer one over the other. A patient with a documented history of acquired B doesn't need a lifelong flag reclassifying their type; they need the discrepancy recognized as transient and resolved on the next clean sample. A post-transplant patient needs active tracking through engraftment, because giving blood matched to their pre-transplant genotype can become the wrong call partway through their course. A weak subgroup needs to be distinguished from acquired B and from genuine group O, because the transfusion consequences of getting that wrong are not symmetric. None of this is exotic. It's the ordinary, unglamorous discipline of not assuming a chart from three years ago is still telling you the truth.

Where sequencing fits

Serology answers what the cells are doing right now. It doesn't always answer why, and in ambiguous cases, that gap matters. Genotyping resolves the categories serology can't cleanly separate: distinguishing a weak subgroup from an acquired phenomenon from early mixed-field chimerism, in a single pass, without waiting for an infection to clear or a repeat sample to confirm a trend. That's the appeal of ABO genotyping as a clinical tool, and it's the part of this space I've been spending the most time in lately. More on that as the work develops.

The identity question

People treat blood type the way they treat a birthday: fixed, inherited, a fact about you that predates memory. Mostly that's fair. But "mostly" is doing real work in that sentence. Somewhere in a transfusion service right now, a phenotype is quietly disagreeing with a genotype, and the discrepancy is not a lab error to be explained away so much as a reminder that even the facts we consider most fixed about our own biology are, on some timescale, conditional.

References

1. Blood Bank Guy Glossary. Acquired B Antigen. bbguy.org.

2. Judd WJ, Friedman BA. The acquired B antigen phenomenon. ASCP Check Sample Program, Immunohematology No. 1-82; 1975.

3. Campbell TA et al. Acquired B antigen: an ABO typing discrepancy successfully reversed by transfusion with type A red blood cells. Transfusion. 1980;20(3):345-348.

4. Resolution of an unexpected ABO typing discrepancy in a 9-month-old patient with juvenile myelomonocytic leukemia. Clin Case Rep. 2020.