Today we differentiate between two basic forms of asthma, allergic and non-allergic asthma. In allergic asthma, an excessive reaction of the immune system leads to chronic allergic inflammation of the respiratory tract. Patients often suffer from other allergic symptoms, such as allergic rhinitis or conjunctivitis (inflammation of the conjunctiva of the eye). In some cases, the symptoms occur only at certain times of the year, e.g. in the case of an allergy to pollen. House dust mite allergy sufferers often have symptoms all year round. Many patients also suffer from non-allergic asthma which is related to non-allergic airway inflammation. It seems however likely that many more subtypes of asthma exist and which differ between children and adults. Yet, reasons for this heterogeneity are not understood.