The unleavened bread used for communion is generally understood to refer to the purity of the sacrifice of Christ. It very well can have connotations of purity, but I wonder if the main reason is different. It seems to me that leavening makes bread alive. When the leaven is missing, bread does not move, but when leaven is present, bread moves and rises because it is alive. Bread is a staple of the body (Mt 4:4, Lk 4:4) and what you eat becomes you, so bread is a good representation of the body. The body of Christ was God’s sacrifice for sin, so bread is a good representation of that sacrifice. I think that unleavened bread represents the fact that the body was dead. Unleavened bread just lays there and does nothing, and hence it is a good representation of a dead body. You can continue the type further to show the death, burial and resurrection. If the bread is a type of the dead body of Christ, when you eat it, it is buried, and when it becomes part of you, it rises to walk in newness of life.