Allergic Cascade
The immune
system is very specific and goal oriented. Although you
may be allergic to a number of substances, allergic
reactions are directed at specific allergens. For example,
you may be allergic to Bermuda grass, but not oysters. At
times, however, two or more foreign substances might appear
similar in nature to the immune system, which may mistake
one for the other and react to both. For example, if you
are allergic to birch trees, your immune system may also
react to apples or other fruits, which it mistakes for
birch pollen. These cross-reactions occur because of
similar allergens that are produced by a variety of
plants. The allergic response, however, is by no means
vague or ill-defined. It is a definite, vigorous attack
aimed, unfortunately, at harmless agents. The end result is
well-defined symptoms and disorders.
The deeper our understanding of the intricate nature of
the allergic reaction, the more likely we are to find more
effective treatments. We need to look more closely at the
chain of events from the initial response to allergens to
the many symptoms that may result. Although misguided, it
is an efficient, well-orchestrated, and potentially
explosive sequence of cellular and chemical interactions.
This is the so-called "allergic cascade."
Who are the "players" in the allergic cascade?
Our body's immune system is designed to constantly be on
the lookout for intruders. It has the remarkable ability
to distinguish between "self" and "non-self" (foreign
substances, which it tirelessly protects us from). Let us
look more closely at this complex process. Take for
example an exposure to ragweed pollen. Once in the body,
the ragweed pollen is engulfed by the immune system's
scouts, the so-called Antigen Preventing Cells or APC's.
These APC's slice up the ragweed pollen into small
fragments, which then combine with special proteins in the
cell, called human leukocyte antigens or HLA's. HLA's
function like a guideline to help the body
distinguish "self" from "non-self." When combined with the
HLAs, the fragments become visible to a key player in the
allergic cascade (the lymphocytes), which recognizes them
as foreign. This ragweed pollen fragment-HLA combination is
exposed on the surface of the APC's in full view of these
specialized white blood cells.
Before we review details of how the various players in
the allergic cascade fulfill their roles, let's note these
basic concepts of types of important cells and messenger
proteins of the immune reaction:
The term white blood cells or leukocytes is derived from
Greek words "leukos" meaning white and "cytes" meaning
cells. The white blood cells are essential to the immune
system and include the monocytes, macrophages, neutrophils,
and lymphocytes.
Lymphocytes are white blood cells that play a key role
in both immunity and allergy. They are divided into two
types, the T and B lymphocytes. Each type is responsible
for a particular branch of the immune system. It is the
duty of the T-lymphocytes to be ready to directly shift
into action to attack foreign substances (cell-mediated
immunity). Some T-lymphocytes are experts at "killing"
(cytotoxic or killer T cells) while others assist the
immune response and are termed "helper" cells (TH cells).
The TH cells are further divided into TH1 (infection
fighters) and TH2 (allergy promoters), depending on the
proteins they release. The partners of the T-lymphocytes
are the B-lymphocytes. B-lymphocytes are tiny antibody
factories that produce antibodies to help destroy foreign
substances when stimulated to do so by the TH cells.
Basophils and eosinophils are other white blood cells
that play an important role in allergy. T cells often call
these cells into action in allergic conditions. Blood
levels of eosinophils are commonly elevated in people with asthma and other allergic diseases.
Cytokines are a diverse group of proteins that are
released by lymphocytes and macrophages in response to an
injury or activation, such as by an allergen. They act as
chemical signals that "step up" or "step down" the immune
reaction.
Next: What about a more detailed look at the "players?" »
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