House dust mite and pet allergies occur because of a reaction to tiny airborne particles within the home. Symptoms can include a runny or blocked nose, itchy, red or watering eyes and wheezing. If you have asthma, your asthma symptoms can also be made worse by these allergies. Regular use of medication can improve symptoms but avoidance of the cause can also be important, especially with pet allergies.
What is the exact cause of house dust mite and pet allergies?
Tiny particles found in the air inside the home that cause allergic symptoms are called aero-allergens. These particles can include the poo (faeces) of house dust mites, and animal saliva, skin and urine. (Mould spores are another type of aero-allergen present in some homes, especially those with damp problems.)
Allergens are substances that cause an allergic response in certain sensitive people. They commonly cause problems with nasal, eye (ophthalmic) and breathing (respiratory) symptoms. These include inflammation of the nose (rhinitis), inflammation of the eyes (conjunctivitis) and wheezing. They can aggravate conditions such as asthma.
Aero-allergens rarely cause severe allergic reactions such as anaphylaxis. They are, however, some of the most common causes of allergies in general. See separate leaflet called Allergy - General Overview for more details.
Treatments do help improve symptoms. Avoidance of these allergens (or at least a reduction in their number) is also important.---
---http://www.patient.co.uk/health/house-dust-mite-and-pet-allergy