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