When examining the "last supper" it is worth considering the ritual within the experience of Romans familiar with their favorite cult; that of the Great Shepherd and Saviour, Mithra.
Paul wrote in Antioch which at the time was a centre of Mithraism and in the first century with no Christian Bible available, Paul was in a position to dictate his personal vision and write it down. Posterity and the Roman Church deciding that his words would constitute the orthodox view.
Mithraism was a mystery cult in other words it was for initiates only, as in any such group the holy 'secrets' would have leaked out and the last supper of Mithras with its symbols for blood and body is a template for the Pauline version. This was part of the synchretising of dogma; the absorption of much of pagan belief and ritual culminating in the all embracing nature or "catholic" belief of the early church.