How did differences over Eucharistic bread contribute to tensions between the Eastern and Western churches?

The use of leavened versus unleavened bread in the Eucharist became a significant point of contention between the Eastern and Western churches, contributing to the growing estrangement that culminated in the Great Schism of 1054.1

While the Eastern Orthodox tradition utilizes leavened bread for the host, the Western church adopted the use of unleavened bread. This liturgical difference was among the specific grievances cited by Cardinal Humbert in the bull of excommunication he placed on the altar of Hagia Sophia in 1054, which condemned the Greek practice as part of a broader conflict over ecclesiastical customs and authority.21

For centuries, these divergent practices served as a visible symbol of the deepening cultural and theological divide between the two traditions. It was not until 1965 that the mutual excommunications were formally abolished by Pope Paul VI and the ecumenical patriarch Athenagoras I, marking a significant step toward reconciliation.1