Why is it that many denominations partake of leavened bread and wine during the Lord's Supper when the bread and wine of the original Supper was unleavened? The Passover coincides with the start of the Feast of Unleavened Bread.
Theologically, Episcopalians liken the leaven to the power of the Holy Spirit giving lift and rise to the loaf. As an Episcopalian myself, however, my pragmatic analysis is that this is primarily because they like the taste better (silly 'Episcopalians). I, too, however dislike the symbolism, because in my mind, leaven is associated with sin.
I don't remember details, but Benedict XVI in his Jesus of Nazareth, volume 2 discussed when did the Last Supper happen and concluded that it's more likely that it has been a day before the first day of the Feast of Unleavened Bread. I don't remember the places in Bible supporting it (and in fact I would rather wait a week before I can re-read the chapter 5 in Jesus of Nazareth for all the reasoning), but Kaiphas wanted to kill Jesus before the feast. There is a tradition (I think it's rooted in Bible but I'm not sure how firmly) that Jesus died in the same hour when the Passover lambs were killed. Also, Joseph of Arimatia hurried with the burial of Jesus "because it was the Jewish day of Preparation" (John 19:42), which makes more sense if the Passover was on Saturday. John's term of Last Supper ("It was just before the Passover Festival." - John 13:1) that the Last Supper was before Passover.
The point is that it's not so clear that Jesus celebrated the Last Supper with unleavened breads. I'll add some more reasoning when I get home and remind what Ratzinger/Benedict XVI writes in Jesus of Nazareth.
Thomas Aquinas in Contra Errores Graecorum (almost in the end) cites some suspicious source claiming to be Pope Gregory saying: “The Roman Church offers unleavened bread because the Word of the Father took flesh without any carnal conmingling; but other Churches offer leavened bread because the Word of the Father is clothed with flesh and is true God and true man. So, also, yeast is mixed with flour and this becomes the true body of our Lord Jesus Christ.”