Cause
When a flea sucks blood from a dog, it passes its saliva into the dog's skin. Dogs that are allergic to flea allergen will experience swelling, irritation, and itchiness. When the dog scratches the bites, further trauma to the skin -- lesions, hair loss, and more intense itchiness -- will result. A single flea bite can cause these symptoms in an allergic dog; animals without this allergy must be bitten many more times before their skin becomes irritated.