Allergic Cascade (cont.)
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What are the consequences of the allergic cascade?
Now that we understand how the allergic reaction develops, let's review the various changes that occur in the body as a result of these early and late phase reactions. When histamine is injected into the skin, a technique used in diagnosing allergies, a reaction that can mimic an allergic reaction occurs. The histamine injection prompts the development of a pale, central swollen area that is caused by fluid leaking out of local blood vessels into the adjacent tissues. This localized reaction is called a "wheal." A red "flare," which sometimes has a warm feeling due to inflammation, surrounds this "wheal." Itching occurs because histamine irritates the nerve endings in the skin.
This early or immediate response peaks at about 15 minutes and fades within 90 minutes. Sometimes, the immediate effects are followed by a late phase reaction that occurs about 4 to 6 hours later and can last up to a day.
Allergens, such as ragweed pollen, react with the tissues lining the inner surfaces (membranes) of the nose and eyes, thereby stimulating mast cells to release chemical mediators, including histamine. The chemical mediators cause a leakage of fluid and the production of mucous, causing a runny nose, itching, and sneezing. The late reaction also causes the tissues to swell and the nose to become congested.
In the lungs, exposure to inhaled allergens causes wheezing, shortness of breath, and coughing within seconds or minutes. These symptoms tend to subside after about an hour. However, after about 4 hours, the late phase reaction can cause a worsening of shortness of breath, wheezing, and coughing. This phase can last for up to 24 hours. The late phase reaction involves an influx of a variety of inflammatory cells to the affected area (eosinophils, neutrophils, lymphocytes, and mast cells) and, if repeated inhalations of allergens cause recurrent reactions, reactions may merge into each other leading to chronic or persistent allergic asthma.
Lastly, allergens can be absorbed into the bloodstream and travel to many sites (including the nose, lungs, throat, skin, and digestive tract), causing multiple symptoms that are typical of a severe allergic reaction (anaphylaxis). Blood vessel dilation may occur throughout the body causing a drop in blood pressure and shock. Although rare, this type of anaphylactic reaction can be caused by medications, insect venoms, and foods.