Food coloring stops the breakdown of proteins, this is a problem. So a protein is broken down into individual amino acids.
You cannot have an immune response to an individual amino acid, but you can have one to an amino acid sequence.
Say you are sensitive to gluten, the quicker you can break that gluten down the better, because it becomes less reactive.
But say you bake that cake with blue food dye to match the birthday balloons, if you are gluten sensitive it is going to knock you on your backside because the food coloring prevents enzymes from allowing the gluten (or any protein) to be broken down into single (unreactionary) amino acids.