Allergy (cont.)
What causes allergies?
To help answer this question, let's look at a common
household example. A few months after the new cat arrives
in the house, dad begins to have itchy eyes and episodes of
sneezing. One of the three children develops coughing and
wheezing, especially when the cat comes into her bedroom.
The mom and the other two children experience no reaction
whatsoever to the presence of the cat. How can we explain
this?
The immune system is the body's organized defense
mechanism against foreign invaders, particularly
infections. Its job is to recognize and react to these
foreign substances, which are called antigens. Antigens are
substances that are capable of causing the production of
antibodies. Antigens may or may not lead to an allergic
reaction. Allergens are certain antigens that cause an
allergic reaction and the production of IgE.
The aim of the immune system is to mobilize its forces
at the site of invasion and destroy the enemy. One of the
ways it does this is to create protective proteins called
antibodies that are specifically targeted against
particular foreign substances. These antibodies, or
immunoglobulins (IgG, IgM, IgA, IgD), are protective and
help destroy a foreign particle by attaching to its
surface, thereby making it easier for other immune cells to
destroy it. The allergic person however, develops a
specific type of antibody called immunoglobulin E, or IgE,
in response to certain normally harmless foreign
substances, such as cat dander. To summarize,
immunoglobulins are a group of protein molecules that act
as antibodies. There are five different types; IgA, IgM, IgG,
IgD, and IgE. IgE is the allergy antibody.
(In 1967, the husband and wife team of Kimishige and
Teriko Ishizaka detected a previously unrecognized type of
immunoglobulin in allergic people. They called it gamma E
globulin or IgE.)
In the pet cat example, the dad and the youngest
daughter developed IgE antibodies in large amounts that
were targeted against the cat allergen, the cat dander. The
dad and daughter are now sensitized or prone to develop
allergic reactions on subsequent and repeated exposures to
cat allergen. Typically, there is a period
of "sensitization" ranging from months to years prior to an
allergic reaction. Although it might occasionally appear
that an allergic reaction has occurred on the first
exposure to the allergen, there must have been a prior
contact in order for the immune system to be poised to
react in this way.
IgE is an antibody that all of us have in small amounts.
Allergic persons, however, produce IgE in large
quantities. Normally, this antibody is important in
protecting us from parasites, but not from cat dander or
other allergens. During the sensitization period, cat
dander IgE is being overproduced and coats certain
potentially explosive cells that contain chemicals. These
cells are capable of causing an allergic reaction on
subsequent exposures to the dander. This is because the
reaction of the cat dander with the dander IgE irritates
the cells and leads to the release of various chemicals,
including histamine. These chemicals, in turn, cause
inflammation and the typical allergic symptoms. This is how
the immune system becomes exaggerated and primed to cause an
allergic reaction when stimulated by an allergen.
On exposure to cat dander, the mom and the other two
children produce other classes of antibodies, none of which
cause allergic reactions. In these non-allergic members of
the family, the dander particles are eliminated
uneventfully by the immune system and the cat has no effect
on them.
Figure 1
The Immune System
Foreign Substance
alt="-"
(cat dander, pollen, virus, bacteria) |
  |
|
Normal Immune Response
IgM, IgG, IgA, IgD and various immune cells respond to attack. |
|
Exaggerated Immune Response IgE is overproduced in response to cat dander, pollens, and other harmless
allergens. |
|
|
|
|
Foreign substance is eliminated. |
Subsequent exposure results in an allergic reaction. |
|
|
|
|
Non-Allergic Individual |
Allergic Individual |
|
Next: Who is at risk and why? »
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