I'm not sure that we know that. Blood typing may not be valid for blood older than about 1000 years. It may test out as AB but is that valid.
Rogers writes: "Several claims have been made that the blood has been found to be type AB, and claims have been made about DNA testing. We sent blood flecks to the laboratory devoted to the study of ancient blood at the State University of New York. None of these claims could be confirmed. The blood appears to be so old that the DNA is badly fragmented. Dr. Andrew Merriwether at SUNY has said that "… anyone can walk in off the street and amplify DNA from anything. The hard part is not to amplify what you don't want and only amplify what you want (endogenous DNA vs contamination)." It is doubtful that good DNA analyses can be obtained from the Shroud."
Dan
Actually, we do know, because of the Eucharistic miracle of Lanciano, another miracle that is impossible to explain.