A previous dog was allergic to food and she had to take Vanectyl to control the itching. Sometimes it was so bad she would chew her pads raw - usually corn prompted this. As she aged, she became sensitive to her environment too, springtime was tough on her.
Present dog also has food allergies. I tried many foods before finding a limited ingredient food (Natural Balance's Bison & Sweet Potato) that does the trick. He's triggered by chicken, beef, lamb, fish -- a wide variety of proteins.
Some say it is not the protein, instead it is the preservative used in the meat before it is processed into dog food. I don't know if that is true, or not.