Blood of Christ in Christian theology refers to (a) the physical blood actually shed by Jesus Christ primarily on the Cross, and the salvation which Christianity teaches was accomplished thereby; and (b) the sacramental blood present in the Eucharist or Lord's Supper, which is considered by Catholic, Orthodox, Anglican, and Lutheran Christians to be the same blood of Christ shed on the Cross.
The Roman Catholic Church, Eastern Orthodox churches, the Oriental Orthodox churches, the Assyrian Church of the East, and Lutherans, together with some Anglicans, believe in the Real Presence of Christ in the Eucharist. The Roman Catholic Church uses the term "Transubstantiation" to describe the change of the bread and wine into the body and blood of Christ. Eastern Orthodox too have authoritatively used the same term to describe the change, as in The Longer Catechism of The Orthodox, Catholic, Eastern Church and in the decrees of the 1672 Synod of Jerusalem.
The Lutheran churches follow the teaching of Martin Luther in defining the presence of Christ in the eucharistic elements as sacramental union (often misconstrued as consubstantiation), meaning that the fundamental "substance" of the body and blood of Christ are literally present alongside the substance of the bread and wine, which remain present. Lutherans too believe in and teach the Real Presence. Most Protestant churches reject the idea of the Real Presence; they observe eucharistic rites as simply memorials.