Think of dog allergies as a bucket under a slow leaking tap. PLUS the bucket itself has a small leak in it.
Depending on how fast the tap leaks the bucket will ultimately "fill" and overflow (think of the overflow as how allergies manifest on the skin/outside of the body).
The stuff that the dog is absolutely completely allergic to (and I mean a REAL 'allergy' not just an intolerance) -- things that aren't going to change like atopic allergies, flea allergy dermatitis -- the "biggies" if you will. Those things are steady and will often keep the body (the bucket) pretty darned full. Doing all we do to help our allergy dogs (bathing, antihistamines or homeopathic/acpuncuture, other alternative treatments) helps keep that bucket from OVER-flowing. And you've got that slow leak at the bottom (things like off-seasons, maturity, good care, good vet care) that also help equalize.
But when the bucket's darned near full and it's "high season" AND the dog gets a ton of "new" potential allergens thrown at it and it's pretty well overflowing ANYWAY, then most any *new* thing you throw at it, is very very apt to react as an allergen.
This is why it can be so dangerous to food switch when you're trying to diagnose -- because yet one more "new" food can often simply be interpreted by the body as yet again one more NEW ALLERGEN instead. Then you run out of 'novel proteins' because you threw too much at the body too fast and it became an allergen rather than something 'ok'.
Ok -- so with care you get that level in that bucket down. The trickle out the bottom is really helping to lower the level and you've got things balanced out so the skin is "ok".
Introducing some new food **very carefully** when everything else is low-key (not high allergy season outside -- maybe introducing just ONE of those ingredients as a topper rather than a whole new food) and you may discover a greater tolerance.
There is such broad mis-conception that food is the ultimate allergen. It's not. But it's often one of those things that will instantly bring on a reaction -- simply because the whole balance of everything else is SO precarious.
In a healthy dog the hole in the bottom of the bucket and the leak INTO the bucket are nearly equal or the one out the bottom is a bit bigger.
People tend to be SO focused on food that they forget that the allergy response in the body is NOT paved with flags that say "Allergic to chicken!!" and anything else is ok. Because the dog is undoubtedly ALSO allergic to a whole host of other things that most folks don't even have a CLUE are allergens. Things like oak, maple or that bush out your back door ... dust, wallpaper, mold and mildew (and maybe your cellar is dirt or something).
People see the reaction with 'food' quicker is all. It's not that they're the ONLY allergens. BUT all the allergens that the body is sensitive to all go into that bucket and pretty well become indistinguishable one from another. They ALL simply push the body's immune system further than it can stretch.
Also -- and this is how it's been explained to me by several vets -- the body's immune system should either be "on" ... or "off". Our immune system should NOT be standing there loaded and rapid firing at any and ALL things.
The immune system should be at 'rest' until the body encounters a real threat. Then the immune system should go "on" -- to destroy that threat. And then the immune system should stand down and be at 'rest' again.
When the immune system becomes over stretched -- and it is maxxed out beyond what it can handle -- it gets stuck in the "on" position.
This is where huge allergic response happens -- because then ANYTHING new is perceived by the body as a potential threat and if too much "new" (whether it's new pollen in the air, or new protein or preservatives in food) is introduced the body simply guns for EVERYTHING.
This is also how auto-immune happens. The body gets so used to being "on guard" that it generalizes so more things generally are seen as 'invaders" -- things that are actually "normal". It's again a result of the body being pushed too hard -- either by toxins or vaccines or medicines or stress of some kind.
Sorry Jetta -- more than you asked for.
But truly -- if you want to try one of those things I wouldn't switch to a whole new food -- but I might try ONE specific thing that you know was a problem previously and try just a bit of it. But I would *not* try it during spring greening, or during house-cleaning or any time when there is stress, schedule changes, or any sort of turmoil.