Environmental allergies and asthma—both are chronic diseases affecting the respiratory tract that tend to cause sneezing, wheezing, and coughing. Though the presence of one often worsens the symptoms of the other, they can remain distinct conditions, and their disease mechanisms are different. Allergies appear in many forms, and reactions can occur throughout the body; inhaled environmental allergens typically trigger reactions and symptoms in the respiratory tract. They are primarily immune reactions to specific allergen exposures that are typically harmless in non-allergic individuals. In comparison, asthma is solely a lung disease, and can be triggered mechanically by various factors…including allergies.
Allergic reactions take many shapes, dependent on the route of allergen entry into the body. Thus, allergies can present with symptoms ranging from hives and itchiness, to difficulty breathing and severe anaphylaxis. The basis for allergic reactions, termed Type I hypersensitivity, is universal for all routes of allergen entry (for instance, intravenous, subcutaneous, inhalation, or ingestion).
The allergic cascade begins with the “priming” of immune cells called mast cells upon an individual’s first exposure to an allergen, such as peanuts or pollen. This is a complex process involving the interaction of many types of immune cells. The allergen is first engulfed by an “antigen-presenting” cell that triggers a cascade leading to the eventual production of antibodies that attach to the surface of “mast cells”. The mast cell is now primed and ready to cause an allergic reaction. When the person is next exposed to the same allergen, it is able to interact directly with the surface antibodies on the primed mast cell, which results in the release of allergic mediators from the cell. These mediators cause potent inflammation—responsible for characteristic allergic symptoms such as redness, itchiness, and swelling—as well as propagate the allergic reaction by attracting even more immune cells to the site of inflammation. When this type of reaction occurs in the upper airway (nose and throat region) in response to inhaled allergens, it is known as allergic rhinitis, a disease characterized by a runny nose, itchy and watery eyes, and sneezing.
Asthma is primarily a chronic lower airway disease and can be induced by various triggers. It involves the narrowing of passages in the lower airway (the bronchi and bronchioles of the lungs), which causes difficulty breathing and wheezing. The air passages of those with asthma are particularly sensitive. They may either become swollen, or narrow due to muscle constrictions. In normal individuals, the muscular walls of the airway are primarily relaxed, but in those with asthma, particular exposures such as cold or exercise precipitate muscle contractions termed bronchoconstriction, making it harder to breathe. Asthma symptoms may also be triggered by viral or bacterial infections or by related conditions such as gastroesophageal reflux disease (GERD).
One type of asthma, however, is often precipitated by the inhalation of allergen. This is called allergic asthma or allergy-induced asthma, and is the most common form of asthma in the population. Many of the symptoms of allergic and non-allergic asthma are the same. Whereas the onset of non-allergic asthma is caused by factors such as those listed previously (cold, exercise, viral infection), allergic asthma is also triggered by inhaling allergens including dust mites, pet dander, pollen (from seasonal plants such as trees, grass, and ragweed), or mold. Thus for those with seasonal allergies, asthmatic symptoms need to be more carefully managed during certain pollen seasons. The inhalation of these allergens triggers the allergic immune cascade outlined above, causing the airways in the lung to become swollen and inflamed. It is this swelling that narrows the air passages and results in difficulty breathing, coughing, wheezing, and other symptoms of asthma.
Though there is no cure for asthma, once diagnosed the symptoms can be effectively managed, such having asthma doesn’t hinder a person’s quality of life. But in order to best control the condition, it is essential to know if one’s asthma is allergic. A specialist such as an allergist or immunologist is best qualified to diagnose the disease, and keeping in mind the intimate link between allergies and asthma, will be able to put a treatment plan in place to best manage individual conditions and lifestyles.
American Academy of Allergy, Asthma & Immunology, Asthma Library located at http://www.aaaai.org/conditions-and-treatments/library/asthma-library.aspx.