It is the protein of this dander that causes the reactions. Dander imbeds itself in the carpeting, furniture and draperies and remains in the environment to cause reactions. During a cat's self-cleaning process, Fel d I protein in the saliva is deposited on the fur. Male cats usually produce more allergen than females. Proteins in the urine of cats will also trigger allergic reactions.
Some animals seem more tolerable than others because they shed fewer allergens.
Unfortunately, there are no non-allergic animals. It is also true that the length of hair does not alter the animal's allergenicity. Within any given breed, or even litter, you may find animals that you can tolerate better than others. Allergies are usually cumulative. Since the allergic person is sensitive to more than one thing, it is the Total Allergen Level that causes a reaction.