It's an interesting topic. There are some studies done with hydrolyzed protein--basically, pre-digested before you even eat them, it protects against diabetes in a rat model of autoimmune type 1.

http://www.ncbi.nlm.nih.gov/pubmed/20853098



I guess this general idea makes sense of why so many of the common allergens are also plant proteins, with a lower digestibility than most animal proteins.

Casein impairing gut barrier is interesting. Maybe the problem with people who have problems with both gluten and casein isn't the one making them more sensitive to the other, so much as a parallel problem, poor digestion of protein causing reactivity and excess zonulin in both cases.

I've also read that casein is more allergenic than whey, though I haven't looked into it enough to vouch for the accuracy. But that also makes sense, bodybuilders advocate both whey and casein, whey because it's quickly digested, so there's more of an acute rise in muscle protein synthesis, casein because it's a much slower digested protein, so better at reducing muscle protein breakdown. So if it really is more allergenic, again there's a protein that's more resistant to digestion being a potential problem.