Catholic doctrine has always held that, upon consecration at Mass, Christ becomes truly and substantially present in the bread and wine on the altar. Over the centuries, however, there have been numerous reports of consecrated Hosts literally turning to physical flesh and blood.
One such miracle happened in 8th Century Lanciano, where a priest who was doubting the Real Presence of Christ in the Eucharist saw the bread and wine transform into human flesh and blood as he said the words of consecration. Over 1,200 years later, these substances have not decomposed.
These substances and others like them produce the same results when tested for blood type. AB. I think this is super cool, because AB is the universal receptor and Jesus took our blood guilt on Himself to save us from eternal damnation.