Allergic Cascade (cont.)
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.
Next: How does understanding the allergic cascade help? »
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