Allergic reactions can become manifest as different diseases, such as seasonal and perennial allergic rhinitis, asthma, atopic dermatitis, chronic urticaria, anaphylaxis and food allergy.

Allergic rhinitis is a common condition in industrialised nations, affecting 10 to 25% of the population. It is characterised by sneezing, itching, runny nose, stuffy nose and itchy/watery eyes. Allergic rhinitis may be further subdivided into seasonal and perennial disease. Seasonal allergic rhinitis (generally called hay fever) is caused by an allergic reaction to pollen released by trees, grasses and herbs at specific times of the year. Perennial allergic rhinitis is triggered by allergens, such as house dust mites or animal dander. These allergens are independent of a particular season so that patients with perennial allergic rhinitis are at risk the year round.

An allergic reaction is a process during which various mediators of inflammation are released.

Histamine is considered the key mediator and often plays a major role in provoking the symptoms of allergies. Not surprisingly, considerable research effort has been focused on the development of compounds that prevent the effects of histamine.

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