After briefly evoking a Black Mass, Mulligan proceeds to mock the mystery of transubstantiation at the heart of the Catholic Mass, literalizing its claim to (literally) reproduce the body of Christ.
Every Mass reenacts the biblical accounts of the Last Supper, in which Jesus symbolically gives his body and blood to his disciples as nourishment. The Catholic Church interprets these symbolic words quite literally. When the priest consecrates the wafer and the wine, they become miraculously transformed into the body and blood of Christ—an ontologically different “substance” into which they are “transubstantiated.”
As Mulligan holds aloft the chalice containing (or about to contain) God's blood, he acknowledges that some of the flecks of shaving foam are experiencing difficulty in their effort to become “white corpuscles,” the antigen-destroying part of the holy substance. When medical students encounter miracles, they like to get their facts straight.