As cats also have an endocannabinoid system, just like us humans they can be allergic to either food or the environment. Allergies are caused by hypersensitivity in the immune system of your cat. It is hard to find the cause of the allergy at best, but the principle behind the allergic reaction is the same. The body reacts aggressively against a regular substance like dust or pollen because it detects it as harmful to the body. The body then reacts by fighting the substance off as if it is a virus or bacteria. This results in the allergic reaction your cat is displaying.