Although many people with allergies to pets think that animal fur is the offending culprit, it's not. Researchers have found that the allergens are actually proteins secreted by oil glands in the pet's skin and shed in its dander. Proteins in saliva and urine are also potent allergens. When the substance carrying the protein dries, it may become airborne. Because they lick themselves so much and are often housebound, cats tend to cause more allergic reactions than dogs do. Proteins in the urine of pet rodents such as guinea pigs, gerbils, mice and rats can also cause allergic reactions.