The doctrine that explains why the whole Christ is present under each Eucharistic species. Christ is indivisible, so that his body cannot be separated from his blood, his human soul, his divine nature, and his divine personality. Consequently he is wholly present in the Eucharist. But only the substance of his body is the specific effect of the first consecration at Mass; his blood, soul, divinity, and personality become present by concomitance, i.e., by the inseparable connection that they have with his body. The Church also says the "substance" of Christ's body because its accidents, though imperceptible, are also present by same concomitance, not precisely because of the words of consecration.
In the second consecration, the conversion terminates specifically in the presence of the substance of Christ's blood. But again by concomitance his body and entire self become present as well. (Etym. Latin concomitantia, accompaniment.)