Itching after a cat scratch is likely due to an allergic reaction to proteins in the cat's saliva, according to WebMD. Outside cats can also bring in pollen, mold and other allergens that can cause an allergic reaction.
Oversensitive immune systems in people with allergies attack substances such as cat dander or saliva proteins as if they were bacteria or viruses. The allergy symptoms are the side effects of the body's fight against the allergen, states WebMD. Symptoms of cat allergies include: coughing and wheezing; hives or rash; red, itchy eyes; runny, itchy nose; and redness of the skin where a cat has scratched, bitten or licked a person.
A less likely reason for itching is cat-scratch disease, a bacterial infection spread by cats, states the Centers for Disease Control and Prevention. The disease spreads when an infected cat scratches a person hard enough to break the surface of the skin. About three to 14 days later, a mild infection can occur at the site of the scratch, which appears swollen and red with lesions and pus. The infected person might also have fever, headache, exhaustion, and swollen, painful lymph nodes near the site of the scratch or bite.
About 10 percent of the U.S. population has pet allergies, states WebMD, with cat allergies twice as common as dog allergies.