Kelly Kearse writes:

Blood typing and the Shroud:
Positive for AB is not the same as AB positive


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The bloodstains on the Shroud are reported to be type AB, as determined forward typing methods. Although semantically similar, being positive for AB blood is not physiologically equivalent to being AB positive. Here’s the difference: blood typing nomenclature typically lists the blood type first (A, B, AB, or O), followed by expression of a molecule termed the Rh factor. The ABO and Rh molecules are not genetically linked and are encoded on separate chromosomes. ABO and Rh are listed together in blood typing because both are important in blood transfusions. (Individuals that are Rh negative can mount an immune response to red blood cells that are Rh positive). You either express the Rh factor or you don’t, which is why the designation positive or negative is used.

A person that is AB positive (AB +) would contain red blood cells that express AB antigens as well as Rh molecules. Relatedly, a person that is AB negative (AB -) would contain red blood cells that express AB antigens, but not Rh molecules. Technically, (semantically), both individuals are positive for AB, but only the first is truly AB positive. Although the blood type of the Shroud is frequently reported as AB positive (or AB negative), to the best of my knowledge, expression of the Rh factor on the bloodstains on the Shroud has never been evaluated. In a personal communication with Baima Bollone last year (through Emanuela Marinelli), he replied that the Rh factor was too degraded for study at the protein level. The positive (and negative) designations that are at times assigned to the Shroud bloodstains are most likely semantic in nature (due to nonstandard use of the words “positive” or “negative”), and not data-driven. It is correct to say that the blood on the Shroud is positive for AB, which is most precisely stated as “types as AB”. It is not accurate to say that the blood on the Shroud is AB positive. This implies additional information about the expression of the Rh molecule, which is unknown