FAD is one of the most common allergies in pets. Pets with FAD are not only irritated by flea bites, but are also allergic to the parasite’s saliva, which contains 15 reactive components. When the pet receives his first flea bite, his immune system responds and sets up a hypersensitivity reaction. The reaction manifests itself as severe itching when the pet is bitten again. This means a bite from a single flea can set off a delayed itch reaction from flea bites received over the past six months. This not only starts a seemingly never-ending itch cycle; it also causes hive-like lesions from all of the bites, making the pet uncomfortable.