Okay, @JAKEISGREAT , here's another long post, but I'll try to keep it simple. LOL!
Food allergies are caused by whole proteins that for some reason a dog fails to break down in the stomach before it goes to the small intestines. This causes the gastrointestinal immune system to react and send histamine soldiers to attack the protein thereby causing allergy flare-up if it gets to be too much. It's like a false alarm gone wild.
Okay, that said, allergy is caused by proteins - which is usually present in large quantities in meat sources. So, just because your dog food is grain-free does not mean it has no proteins that your particular dog can't handle!
Now, why grain-free is better in dog food and how it relates to allergies...
Dog's digestive system does not process gluten (proteins found in grains) efficiently. Therefore, a dog's stomach has a higher chance of passing gluten to the small intestine intact causing the immune system to send the soldiers... But, not all dogs have a problem breaking down gluten, so why is grain-free still considered better? That is because, a dog doesn't have the proper enzymes in their digestive fluid that can break down carbs present in grain (and all other carb sources in the dog food) either. So, all that carb goes unprocessed straight into the pancreas - the only place that has the enzyme that can break it down. So that, a dog eating lots of carbs are taxing their pancreas. But, sure the pancreas can handle lots of carbs, then what? Where does all the sugar (broken down carbs) go? They get converted to glucose to provide a dog energy in addition to the proteins... but the body stores glucose first before it stores extra proteins so that a dog can easily get obese from a high carb diet. So here you got a triple whammy - not only does the grain tax the dog's digestive system as it tries to break down gluten, it also taxes the pancreas excessively as it tries to break down carbs, then it provides excess glucose that can lead to obesity.
This is really a common misconception, even here on EBN, that a dog food needs to be lower in protein to prevent obesity. BUT - a lower protein dog food usually means it is higher in carbs! So, you're doing nothing but swapping a properly-dog-digestible calorie for another calorie that a dog is ill-equipped to handle. If your dog is getting too fat, don't swap protein for carbs. Instead, lower the calories - which usually means, feed them less.
Okay, back to allergies... if your dog is exhibiting food allergies, an easy way to reduce the amount of guess work on what could possibly be causing it is to get an allergy test done. Another method is to do a food elimination diet. That is - if he's allergic to the Fromm Grain-free Duck flavor, then choose a different protein source - like the Fromm Grain-Free Beef (if you're just changing flavors and not formula - like it's still within the same Fromm Grain-free line, you won't need to go through a slow transition). If that doesn't work, then go to a limited ingredient diet - very few protein sources and swap proteins if the symptom persists.
Hope this helps and not too confusing!
P.S. Proteins are not only found in meat...