In contrast to the “major” crossmatch (recipient serum vs. donor red blood cells), the “minor” crossmatch is designed to test opposite compatibility: The donor’s serum/plasma with the recipient’s red cells. For example, to perform a minor crossmatch between a group A blood donor and a group B recipient, blood bank personnel would mix the donor’s plasma/serum (containing anti-B antibodies) with the recipient’s red blood cells. The two, of course, would be incompatible. Obviously, my example is silly, because no one would deliberately try to give group A blood to a group B recipient, but it illustrates the principle.
Minor crossmatches are rarely performed, for two main reasons: First, transfused blood is screened for unexpected (non-ABO) antibodies, so performing a minor crossmatch to make sure a non-ABO antibody won’t cause a problem doesn’t make a lot of sense. Second, since the volume of transfused plasma is generally small in comparison to the patient’s blood volume, minor incompatibilities (even if not detected by the donor antibody screen) are not of great consequence. For example, a group O red blood cell transfusion to a group A recipient would have a minor crossmatch incompatibility, but is not contraindicated due to the very small amount of incompatible antibody present in the unit of red cells.
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