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Pathology and causes of Allergic Rhinitis

Rhinitis refers to the inflammation of the nasal membranes; it may be acute or chronic. The most widespread form of infectious rhinitis is the common cold, which is caused by virus. Allergic rhinitis, commonly known as nasal allergy, is a type of allergy. It happens when the human body overreacts to specific allergens. Symptoms include chronic or repeated sneezing, runny nose, nasal congestion, and itchiness.

Allergic rhinitis is the allergic reaction of a sensitive individual to specific allergens. Its symptoms, apart from those similar to those of rhinitis, are itchy eyes, throat and ears. If a patient is exposed to an allergen, the allergen will react with the antibody IgE in the blood and bind together the allergen and the receptors on the mast cell, which causes an allergic condition. If he/she comes into contact with the same allergen again, the IgE molecules in the vicinity will cross-link and cause the mast cells to break down, releasing different substances like histamine, leukocyte chemotactic factor or prostaglandin. Histamine can lead to allergic reactions or inflammation and in turn produce basic symptoms of nasal allergy including sneezing, itchy nose and runny nose.

There are two types of allergic rhinitis: seasonal allergic rhinitis and perennial allergic rhinitis. Seasonal allergic rhinitis is also known as hay fever, which is often caused by pollen and mould, and is most common in spring and summer when it is warm and humid. Perennial allergic rhinitis is triggered by allergens like dust mites, animal dander, and medicines.

The incidence rate of this disease has increased substantially in recent years. It is as high as 40% in New Zealand and Australia. Statistics show that 0.8% to 14.9% of children between 6 and 7, 1.4% to 39.7% of teenagers between 13 and 14, and an average of 16% of adults suffer from allergic rhinitis. Some of the areas with a low incidence are Indonesia, Albania, Romania and Greece; those with a high incidence include Australia, New Zealand, UK, USA, Canada and Hong Kong. The average incidence rate around the world is 13.9%; nearly half of these patients have both asthma and eczema. Where there are fewer cases of allergic rhinitis, asthma and eczema are also less common.

There are many causes leading to allergic rhinitis. It may be genetic factors, environment factors, bacterial and fungal infections, medicine containing aspirin, allergen-filled workplace and household environment, puberty, pregnancy, menstruation, or hormonal imbalance. All these may trigger nasal allergy. Most patients will become sensitive gradually if they are exposed to allergens repeatedly within a short period of time. Allergic reactions only occur during the first time an individual is exposed to a large amount of a certain allergen. As the disease develops, even a small amount of the allergen will trigger the reactions. 

If untreated, nasal allergy can worsen the conditions of other related diseases or complications such as sleep apnoea, asthma, infectious rhinitis, sinusitis, and tympanitis.