My dog has bad allergies and I ran across dog food that was labeled no ruminants. Is that in all dog feed?what does it do for a dog. Is it needed for them to lead a healthy life. I have a feelin this may be what he is allergic to.

Anyone familiar with Purina HA (prescription)? The vet just prescribed this for my boston terrier.
The reason I ask is, I’ve read prior posts saying dogs with allergies should stay away from foods with corn starch and soy….and low and behold, the first two ingredients on Purina HA’s ingredient list are “corn starch” and “modified isolated soy protein.” My dog was eating Beneful up until about a month ago, and then we switched to Canidae, which is supposed to be all natural.
Anyone here have any experience with Purina HA? I’m really confused, and it’s driving me crazy to see my dog suffer. Someone help!! Thank you.