Objection 1. It seems that the Paschal Lamb was not the chief figure of this sacrament, because (Psalm 109:4) Christ is called "a priest according to the order of Melchisedech," since Melchisedech bore the figure of Christ's sacrifice, in offering bread and wine. But the expression of likeness causes one thing to be named from another. Therefore, it seems that Melchisedech's offering was the "principal" figure of this sacrament.
Objection 2. Further, the passage of the Red Sea was a figure of Baptism, according to 1 Corinthians 10:2: "All... were baptized in the cloud and in the sea." But the immolation of the Paschal Lamb was previous to the passage of the Red Sea, and the Manna came after it, just as the Eucharist follows Baptism. Therefore the Manna is a more expressive figure of this sacrament than the Paschal Lamb.
Objection 3. Further, the principal power of this sacrament is that it brings us into the kingdom of heaven, being a kind of "viaticum." But this was chiefly prefigured in the sacrament of expiation when the "high-priest entered once a year into the Holy of Holies with blood," as the Apostle proves in Hebrews 9. Consequently, it seems that that sacrifice was a more significant figure of this sacrament than was the Paschal Lamb.
On the contrary, The Apostle says (1 Corinthians 5:7-8): "Christ our Pasch is sacrificed; therefore let us feast... with the unleavened bread of sincerity and truth."
I answer that, We can consider three things in this sacrament: namely, that which is sacrament only, and this is the bread and wine; that which is both reality and sacrament, to wit, Christ's true body; and lastly that which is reality only, namely, the effect of this sacrament. Consequently, in relation to what is sacrament only, the chief figure of this sacrament was the oblation of Melchisedech, who offered up bread and wine. In relation to Christ crucified, Who is contained in this sacrament, its figures were all the sacrifices of the Old Testament, especially the sacrifice of expiation, which was the most solemn of all. While with regard to its effect, the chief figure was the Manna, "having in it the sweetness of every taste" (Wisdom 16:20), just as the grace of this sacrament refreshes the soul in all respects.
The Paschal Lamb foreshadowed this sacrament in these three ways. First of all, because it was eaten with unleavened loaves, according to Exodus 12:8: "They shall eat flesh... and unleavened bread." As to the second because it was immolated by the entire multitude of the children of Israel on the fourteenth day of the moon; and this was a figure of the Passion of Christ, Who is called the Lamb on account of His innocence. As to the effect, because by the blood of the Paschal Lamb the children of Israel were preserved from the destroying Angel, and brought from the Egyptian captivity; and in this respect the Paschal Lamb is the chief figure of this sacrament, because it represents it in every respect.
From this the answer to the Objections is manifest.
Source: Summa Theologica (New Advent)