Christ’s Passion, Thomas says ( ST III, 48, 3), doesn’t seem to be a sacrifice: “human flesh was never offered up in the sacrifices of the Old Law” and were indeed condemned (citing Psalm 105:38).
Thomas replies by emphasizing the figural character of the Old Covenant. Christ’s death was prefigured or typified in the sacrifices of the Old Law, but the similarity is not total: “truth must go beyond the figure.” It is the most perfect sacrifice because, being flesh, it’s offered for men and to them at the Table; because it is passible and therefore “fit for immolation”; because, being sinless, it can cleanse sin; because, being Jesus’ own flesh, it is acceptable to God. Nothing could be more appropriate than “this immolation of mortal flesh.”
The structure of the answer is right, but the substance doesn’t answer the objection. That is, Christ’s death is the sacrifice prefigured in the Old Law; but Thomas doesn’t provide a persuasive rationale for saying that the antitype must be a human self-sacrifice. To make such a case, he would have to attend to some of the details of the sacrificial system - the role of the aqedah as the foundation and background for all sacrifice; the Levitical designation of sacrificial animals as “sons”; the analogies between sacrifices and priests in Leviticus 21-22. When these details are taken into account, there are clues and hints woven throughout the system that point to the sacrifice of a second Isaac, a son of Abraham.