This question only confuses people because they think that if something gives any result on a blood type test it must be human blood. But that is not the case.

To quote Tesoriero’s current favourite specialist, Dr Zugibe, “[I]t is well known that false positives are common in typing ancient blood due to many factors, including antigens from other organisms such as insects.[1]” He further quotes Dr. Alan Adler saying “a lot of old materials test AB because it can be confused with bacterial cell-wall contamination. You get immunological tests for two reasons: the proteins have stretches of sugars, so-called saccharides, that the common blood antibodies react to, and the cell wall of bacteria are in fact saccharides.” Dr. Zugibe was talking about very old blood on the Shroud of Turin, but the same principle applies. Contamination from other organisms, including animals, plants, fungi and bacteria, can lead to false positives on a blood test.

An ABO blood type test isn’t a test for blood. The test assumes that you already know that the sample is uncontaminated blood and are just trying to determine what type it is.

Humans do not develop antibodies to ABO antigens so that we will die if we got a blood transplant from a mismatched donor. The purpose of antibodies is to protect us from pathogens, and we develop these antibodies because they are similar to antigens found on things that can hurt us, like bacteria. That’s why our immune system learned to attack them. As a result, lots of organisms (or bits of dead organisms) will give a “blood type” result on an ABO blood test. This includes bacteria, viruses, fungi, plants, and other animals. People can even develop an “acquired blood type” due to a bacterial infection which will cause them to get different results on a blood test than would be expected from their genes.

Serratia maracens, the bacteria that likely causes the red colour in most of the rotting communion wafers, all by itself, gives a B blood type on an ABO blood test. To get AB, some other organism would also have to be present that produces a protein sufficiently similar to the human A antigen to be recognized by human anti-A antibodies. But a starchy communion wafer left in water for weeks will naturally have a whole community of microorganisms growing on it. Most of the likely candidates haven’t been tested for blood type, because these tests are done to determine which contaminants might affect forensic blood typing, so they tend to focus on things that grow on corpses not things that grow on wet bread. However, given how many different organisms are likely to be growing on any given sample, it wouldn’t surprise me if nearly every communion wafer left in water for weeks would test as having blood type AB, whether it turned red or not.

[1] Zugibe, Frederick T., The Crucifixion of Jesus: A Forensic Inquiry, Second Edition, 2005