Immune System Series
Disorders of the Immune System: Allergy
The most common types of allergic reactions-hay fever, some kinds
of asthma, and hives-are produced when the immune system response
to a false alarm. In a susceptible person, a normally harmless substance-grass
pollen or house dust, for example-is perceived as a threat and is
attacked.
Such
allergic reactions are related to the antibody known as immunoglobulin
E. Like other antibodies, each IgE antibody is specific; one reacts
against oak pollen, another against ragweed. The role of IgE in
the natural order is not known, although some scientists suspect
that it developed as a defense against infection by parasitic worms.
The
first time an allergy-prone person is exposed to an allergen, he
or she makes large amounts of the corresponding IgE antibody. These
IgE molecules attach to the surfaces of mast cells (in tissue) or
basophils (in the circulation). Mast cells are plentiful in the
lungs, skin, tongue, and linings of the nose and intestinal tract.
When
an IgE antibody siting on a mast cell or basophil encounters its
specific allergen, the IgE antibody signals the mast cell or basophil
to release the powerful chemicals stored within its granules. These
chemicals include histamine, heparin, and substances that activate
blood platelets and attract secondary cells such as eosinophils
and neutrophils. The activated mast cell or basophil also synthesizes
new mediators, including prostaglandins and leukotrienes, on the
spot.
It
is such chemical mediators that cause the symptoms of allergy, including
wheezing, sneezing, runny eyes and itching. They can also produce
anaphylactic shock,
a life-threatening allergic reaction characterized by swelling of
body tissues, including the throat, and a sudden fall in blood pressure.
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