Some have said that Christ during the supper gave His body and blood to His disciples, but did not partake of it Himself. But this seems improbable. Because Christ Himself was the first to fulfill what He required others to observe: hence He willed first to be baptized when imposing Baptism upon others: as we read in Acts 1:1: "Jesus began to do and to teach." Hence He first of all took His own body and blood, and afterwards gave it to be taken by the disciples. And hence the gloss upon Ruth 3:7, "When he had eaten and drunk, says: Christ ate and drank at the supper, when He gave to the disciples the sacrament of His body and blood. Hence, 'because the children partook [Vulgate: 'are partakers' (Hebrews 2:14)] of His flesh and blood, He also hath been partaker in the same.'"
Thomas Aquinas, ST 3.81.1.