When an allergen such as pollen or dust is inhaled by an individual with a sensitized immune system, it triggers antibody production. These antibodies mostly bind to mast cells, which contain histamine. When the mast cells are stimulated by pollen and dust, histamine (and other chemicals) are released. This causes itching, swelling, and mucus production. Symptoms vary in severity between individuals. Very sensitive individuals can experience hives or other rashes. Particulate matter in polluted air and chemicals such as chlorine and detergents, which can normally be tolerated, can greatly aggravate the condition.
Sufferers might also find that cross-reactivity occurs. For example, someone allergic to birch pollen may also find that they have an allergic reaction to the skin of apples or potatoes. A clear sign of this is the occurrence of an itchy throat after eating an apple or sneezing when peeling potatoes or apples. This occurs because of similarities in the proteins of the pollen and the food. There are many cross-reacting substances.
Some disorders may be associated with allergies: Comorbidities include eczema, asthma, depression and migraine.
Allergies are common. Heredity and environmental exposures may contribute to a predisposition to allergies. It is roughly estimated that one in three people have an active allergy at any given time and at least three in four people develop an allergic reaction at least once in their lives. The two categories of allergic rhinitis include:
- Seasonal – occurs particularly during pollen seasons. Seasonal allergic rhinitis does not usually develop until after 6 years of age.
- Perennial – occurs throughout the year. This type of allergic rhinitis is commonly seen in younger children.
On examination, persons with allergic rhinitis have characteristic physical findings that include conjunctival swelling and erythema, eyelid swelling, lower eyelid venous stasis (allergic shiners), lateral crease on the nose (allergic crease), swollen nasal turbinates, and middle ear effusion.