Would a person with Celiac disease be protected by transubstantiation from being harmed by gluten in the host?
The argument that the accidental qualities of bread cannot harm the intestine of one who suffers from Celiac disease (due to non-tolerance of gluten in wheat bread) is false. It is of course true that the substance of the bread does not remain after the consecration of the sacred species. However, all the accidents remain, which include not just the exterior appearance, but everything that is subject to the senses and that science can investigate, including the chemical composition. The chemical effects of the gluten on the intestinal wall will consequently still remain, just as much as the appearance and texture of bread, for they are just as accidental to the real nature of what is there as the appearance and texture. Here lies the miracle and the mystery of the Blessed Eucharist. It would be a miracle if the accidental qualities of gluten were not to harm the intestine. Although such miracles can happen, we cannot depend upon such an extraordinary intervention of Almighty God. Consequently, a person who suffers from Celiac disease needs to ask the priest to give him or her a very small portion of the host. It is never allowed to manufacture the host out of rice or a non-wheaten material that does not contain gluten. Such hosts are not valid matter for the Holy Eucharist. [Answered by Fr. Peter R. Scott]
Well said. In churches which offer both species to the laity during Communion, it may be prudent for those sufferring from Celiac disease to commune under only the species of wine, just as perhaps those recovering from alcoholism ought to commune under only the species of bread. [If you're not familiar, by species of bread, I mean Jesus Christ under the appearance of bread, and with the material properties thereof]. This makes sense: after all, you're not obliged to take the Eucharist is it's been poisoned, etc. - Question 3 here addresses what to do in those sort of worst-case possibilites (in short, you're supposed to dilute the poisoned Eucharist in water until it no longer contains recognizable species of bread water). Obviously, an allergy to wheat isn't the same as a foreign poison being added, but Celiac disease is potentially fatal, so the same prudential judgment ought to apply.
However, there's a catch. The priest celebrating the Mass is required to receive under both species. Which has mean that those with Celiac disease have an additional barrier to the priesthood, since that disease tends to worsen over time. Thus, the Church has cautioned bishops to "proceed with great caution before admitting to Holy Orders those candidates unable to digest gluten or alcohol without serious harm." That's because to validly consecrate the bread and wine, it must be actual bread and wine. No Zima, no grape juice, and no fake-bread. To count as bread, it must contain wheat. So what to do about the issue?
Enter Sister Lynn Marie D'Souza and Sister Jane Heschmeyer of the Benedictine Sisters of Perpetual Adoration. As you can probably guess from that funky last name, I'm a bit biased here (Sr. Jane is my aunt). Aunt Jane (calling her "Aunt Sister Jane" sounds creepy) had been working on trying to design a Celiac-friendly host, when Sr. Lynn, with a background in biomedical science, joined the postulancy. Together, the Holy Spirit worked through them to create a type of host which contains only .01% gluten, safe enough for almost everyone suffering from Celiac disease. PBS interviewed them, and there's a 6-minute video here that's pretty cool. Deo Gratias!