One of the most beautiful and intimate ways we participate in the Mass is through the reception of Holy Communion, the Body and Blood of our Lord Jesus Christ. The General Instruction on the Roman Missal asserted the “meaning” of Communion is most clearly signified when given under both species— both the Sacred Host and the Precious Blood. Here the imagery of receiving the Body and Blood of Christ, of being joined with the Messianic Banquet, becomes most clear (#240). However, no where has the Church mandated that both species must be offered.
We must remember that “the whole and entire Christ and the true sacrament are received under either species” (Council of Trent, Doctrine on Communion Under Both Species and on Communion of Little Children). If one receives either the Precious Blood, or the Sacred Host, or both, one receives the fullness of the grace of the sacrament. This doctrine became very clear to me when I was a deacon. At my assignment in Philadelphia, I visited an elderly lady who had lost much of her mouth to cancer. She had a permanent feeding tube through which she poured liquid nourishment. When I brought her Holy Communion, I would bring a vial of the Precious Blood which had been consecrated at the morning Mass and which she then poured down the tube, followed by water for cleansing. Did she receive Christ? Absolutely. Did she receive as much grace as those who received the Sacred Host? Absolutely.
Nevertheless, in 1970, the Sacred Congregation for Divine Worship allowed the local conferences of bishops to determined as to what extent, what motives, and what conditions Holy Communion could be recieved under both species. The General Instruction had already listed several circumstances where communion under both species was permitted, for example to the bride and groom at a nuptial Mass or the assisting extraordinary Ministers of the Eucharist (#242).
In 1984, the National Conference of Catholic Bishops in America decided that the policy was left to the local ordinary of each diocese. Each bishop must weigh the spirtual needs of his diocese with any practical concerns.