The most common types of allergic reactions-hay
fever, some kinds of asthma, and hives-are produced when the immune
system responds 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 sitting 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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