Jesus instituted the communion during the Feast Of Unleavened Bread (Matt. 26:17-29), for this, and at least one other reason, we make our communion loaves from unleavened bread. Does it have be unleavened though? Most preachers I have talked to agree that just because Jesus used unleavened bread during the Feast of Unleavened Bread isn't the only reason; he also was in a rented upper room (Luke 22:12) and we don't do that. The main reason is that they say yeast is a symbol of corruption. They base this idea on several passages such as Matt. 16:6, 11 "be on your guard against the yeast of the Pharisees and Sadducees," and 1Cor. 5:6-7 "Don't you know that a little yeast works through the whole batch of dough? Get rid of the old yeast that you may be a new batch without yeast...." The idea that yeast is a symbol of corruption breaks down when we also read "The kingdom of heaven is like yeast that a woman took and mixed into a large amount of flour until it worked all through the dough" (Matt. 13:33). Rather than being a symbol of corruption, yeast seems to be a symbol of something that quickly spreads.
It is also interesting that our preachers forbid using baking powder to leaven the loaf, for the above reasons. Even though baking powder isn't yeast and wasn't invented back then they feel that any kind of leavening is a form of corruption. That seems kind of reasonable except when you read cookbooks that have sections like "Leavening by steam" in Shirley Corriher's book Cookwise (page 77). As she says "Steam is a powerful leavener," and as many of our sisters can tell you a true sponge cake does not use any leaven other than water. If we can't use baking powder because it is form of leaven and any kind of leaven is a symbol of corruption than we should leave out water when we make our communion bread. As our preachers like to say during debates "that which proves too much, proves nothing."
One more reason for questioning whether we have to only use unleavened bread for the communion loaf is the fact that "unleavened" is never used in reference to the communion loaf. The word that is used is artos, it refers to any kind of bread. If you wanted to talk about unleavened bread you would use the word azumos. Azumos is never used to discuss the communion loaf, only artos is used. Paul, when he was telling the Corinthian brethren how to do the communion correctly, used the word artos for the loaf. Why didn't he use azumos? Maybe because it doesn't matter what kind of bread we use.
I have no problem using unleavened bread, and I have no problem using it because it is what we have always used. I do have a problem saying we have to use it because that is what the bible says we have to use. Speak where the bible speaks and be silent where the bible is silent.