Catechism of the Catholic Church: 1322-1323
Article 3 THE SACRAMENT OF THE EUCHARIST

1322  The holy Eucharist completes Christian initiation. Those who have been raised to the dignity of the royal priesthood by Baptism and configured more deeply to Christ by Confirmation participate with the whole community in the Lord's own sacrifice by means of the Eucharist.
1323  "At the Last Supper, on the night he was betrayed, our Savior instituted the Eucharistic sacrifice of his Body and Blood. This he did in order to perpetuate the sacrifice of the cross throughout the ages until he should come again, and so to entrust to his beloved Spouse, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a Paschal banquet 'in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us.'"<sup>133

133</sup> SC 47.

What is the Eucharist?  It is the bread and wine that by the words of consecration have become the Body and Blood of Jesus Christ.  It is Christ himself, the eternal high priest of the New Covenant who, acting through the ministry of his priests, offers the Eucharistic sacrifice.  And it is the same Christ, really present under the form of bread and wine, who is the offering of the Eucharistic sacrifice.  Christ is both the priest and the sacrificial lamb, priest and victim.

When the bread and wine are consecrated in the Eucharist, they cease to simply be bread and wine, and become (respectively), the body and blood of Christ.  This inner transformation is referred to as " transubstantiation."  Transubstantiation means that while the Body and Blood still retain the outward physical properties of bread and wine, at the words of consecration, their very substance is permanently changed so that they become in fact, the Body and Blood of Christ.  (CCC 1374-76)

For this reason, the consecrated hosts that are not consumed at any given Mass are reserved in a special retaining vessel known as the "tabernacle."  Recall in the Old Testament that the Israelites kept the sacred tablets among them in a tent as a sign of God's presence among them.  The tabernacle is made a holy object by the content which it holds: Christ himself.  Reserved consecrated hosts are later distributed to the sick and elderly.  The Precious Blood because of the properties of wine that will spoil, must be consumed entirely at the Mass in which it was consecrated. 

Consecration of the bread and wine represents the separation of Jesus' body from his blood at Calvary.  However, Christ has risen, and as such, the body and blood of Christ can no longer be truly separated.  Where one is, so the other must be. Therefore the person receiving either the Sacred Host or Precious Blood is receiving Christ, whole and entire.  

The importance of this - the receipt of the entire Christ regardless if the body or blood is consumed - is important since individuals with aversions to gluten or fermented beverages can still receive the entire Eucharist by accepting either body or blood.

Proper Reception of the Eucharist:
Because we are receiving Christ himself at the Eucharist, Catholics must be in a " state of grace."  A state of grace means that we are free of any serious sin, and living according to the teachings of the Catholic Church.  For adults, this includes if we are married, we are married by the Church.  If we are single, we are living chastely.  (CCC 1384-1395, 1415)

If one is conscious of grave sin, he or she must make amends to correct the situation and seek forgiveness through the Sacrament of Reconciliation (Confession) prior to reception of the Eucharist. 

We are to fast for at least one hour prior to the reception of the Eucharist.  Fasting is from food or beverage, including gum, and candy.  Drinking water or taking necessary medication is permitted.