Cats can develop allergies to virtually any cat food ingredient. Allergies can be manifested as skin problems, specifically severe itching. Scratching can result in significant hair loss. If a cat develops an allergy, it is important to identify and eliminate the responsible ingredient. Your veterinarian, or a veterinary dermatologist, can perform sensitivity testing to identify the ingredient containing the allergen. Several commercial diets are available containing "novel" ingredients that are unlikely to cause allergies in your cat.