In medicine, animal allergy is hypersensitivity to certain substances produced by animals, such as the proteins in animal hair and saliva. It is a common type of allergy.
Allergies are caused by an oversensitive immune system, leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria and viruses. Allergy occurs when the immune system reacts to substances (allergens) that are generally harmless and in most people do not cause an immune response.