Back in October, I did write something on the subject as a by-the-way comment to a much longer off-topic posting on evolution, Open Letter to Another Blogger. I wrote:
BTW: You make a good point about blood type AB. Al Adler, a blood specialist from Western Connecticut State University, and another Shroud scientist, pointed out that all old blood tended to test AB because the compounds that generated the test response were also in the cell walls and if the walls degraded the blood started to test AB. But it was possible, he felt, to discern false AB positive readings from real AB type readings.
I wish I knew more about the subject.