Short answers, pulled from the story.
The Eucharist is a Christian rite in which consecrated bread and wine are consumed by worshippers. It is considered a sacrament in most churches and an ordinance in others. Christians believe it was instituted by Jesus Christ at the Last Supper, the night before his crucifixion, when he commanded his followers to repeat the act in his memory.
The word Eucharist comes from the Greek noun eucharistia, meaning "thanksgiving." The related verb eucharistesas appears several times in New Testament accounts of the Last Supper. The term was used for the rite in the Didache, a late 1st or early 2nd century document, and in the writings of Ignatius of Antioch and Justin Martyr.
Transubstantiation is the Catholic teaching that the entire substance of the bread and wine is changed into the body and blood of Christ during consecration, while the appearances of bread and wine remain unaltered. The Fourth Council of the Lateran used the term in 1215, and the Council of Trent declared it definitively in 1551.
Catholics teach transubstantiation, holding that the bread and wine become the actual body and blood of Christ. Lutherans teach the sacramental union, affirming Christ is present "in, with, and under" the forms of bread and wine. Reformed Christians describe a real but spiritual presence. Anabaptists and memorialists hold the Eucharist to be only a symbolic remembrance with no change in the elements.
Open communion allows any baptized Christian, or in some churches any person, to receive the Eucharist. Closed communion restricts reception to members of the same denomination or congregation. The Catholic Church and Eastern Orthodox churches practice closed communion under normal circumstances, while the United Methodist Church practices open communion, inviting all who intend a Christian life.
The gluten in wheaten communion bread poses a risk to people with celiac disease. The Catholic Church addressed this in a letter dated the 24th of July 2003, permitting low-gluten bread for celiacs but not fully gluten-free bread. A study of 681 individuals found that daily communion from a shared cup did not increase infection risk, and no case of disease transmission from a common communion cup has ever been formally documented.