Causes of allergies and sinusitis: rhinitis, sinusitis, mechanisms of respiratory allergy

Mechanisms of Respiratory Allergy 
Symptoms of allergic rhinitis / Symptoms of asthma / Interaction of heredity and environment / Bronchial irritability/ Allergy - bronchial irritability relationships 
 

RHINITIS

Causes of symptoms of allergic rhinitis.   Symptoms  are caused by excessive secretions of mucus glands in the nose, congestion of large veins (venous sinusoids) in the nasal cavity that causes obstruction to nasal airflow, and irritation of sensory nerves in the nose, throat and eyes by allergic inflammation.  These symptoms occur when airborne allergens are inhaled.  These allergens are harmless unless the body's immune system reacts to them by making antibodies to them.   In allergic reactions, these antibodies are in the IgE class of antibodies, which stick to the surface of specialized cells called mast cells.  Allergens combine with IgE antibodies to release  substances including histamine, leukotrienes and cytokines from mast cells and other cells in the lining of the nose.  These substances cause inflammation and symptoms of allergy.

ASTHMA

Causes of symptoms of asthma.   Symptoms are caused by excessive secretions in the bronchi, spasm of smooth muscle in the bronchial wall, and inflammatory swelling of the bronchial lining (mucosa).  These changes cause obstruction to airflow in and out of the lung and the increased obstruction during exhalation leads to wheezing  and trapping of air in the chest. 

Interaction of heredity and environment.   The tendency to become allergic is inherited.  For example, over 50% of children whose parents both have allergic disease will also develop allergic disease.  Allergy is controlled by several genes that influence production of IgE antibodies.  Allergic disease develops only when a person becomes exposed to those allergens to which he has a genetic predisposition.  There is increasing evidence to suggest that predisposition to respiratory allergy may also be increased by protection from bacterial exposure in early childhood (such as aggressive antibiotic therapy of respiratory infection, and avoidance of exposure to animals). 

Mechanisms of bronchial irritability.   A fundamental problem in asthma is irritability of bronchi caused by inflammation in the bronchial walls.  The inflammation causes loss of protective epithelial cells from the mucosa (lining of bronchial wall), exposing sensitive nerve endings to the irritating effects of chemical air pollutants and dry air.  Predisposition to asthma is inherited, probably as the result of multiple genetic factors some of which are independent of allergy-determining genes.  Recent research suggests that genes that control various inflammatory mechanisms are important in causing allergic inflammation in the lung.

Relationships between allergy and bronchial irritability.  Allergic reactions in the lung cause inflammation, during which inflammatory cells called eosinophils and lymphocytes are attracted to the bronchial tissues.  Products from these cells cause damage to the bronchial lining, which in turn leads to bronchial irritability that persists for several weeks after withdrawal from allergen exposure. This is most likely to occur when there is a delayed response to allergen known as a late phase reaction.   Prolonged allergic inflammation in the bronchial walls may lead to thickening of the supportive membrane under the epithelial lining of the bronchial airways, and to a gradual and progressive loss of the ability of the airways to respond to a bronchodilator drug.

For additional information on how abnormalities of the immune system cause allergic reactions, follow these links to the American Academy of Allergy Asthma and Immunology:
What is an Allergic Reaction?

Additional information on asthma
Asthma triggers and management
Exercise-induced asthma
Occupational asthma
Asthma in pregnancy
Rachelefsky G, Garrison P: Free your child from asthma.  McGraw Hill, New York, 2006

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Comments and suggestions welcome!   Email: schumach@u.arizona.edu
Content Owner:  Michael J. Schumacher, MB, FRACP, The University of Arizona

Updated 7/2008