Allergic Cascade (cont.)
What are cytokines?
Cytokines are small proteins that can either step-up or
step-down the immune response. One of the cytokines,
interleukin 4 (IL4), is essential for the production of
IgE. Interleukin 5 (IL5) and others are important in
attracting other cells, particularly eosinophils, which
then promote inflammation. This spectrum of cytokines is
also released by the TH2 lymphocytes, thus further
promoting allergic inflammation.
What is the "early phase" of an allergic reaction?
We have seen how the first encounter with ragweed
pollen sensitizes the body with the help of lymphocytes
and results in the IgE coating of the mast cells and
basophils. Subsequent exposure results in the immediate
release of the chemical mediators that cause the various
symptoms of allergy. This process is the "early phase" of
the allergic reaction. It can occur within seconds or
minutes of exposure to an allergen. This is also known as
an immediate hypersensitivity reaction, which in this case
is to the ragweed pollen allergen.
In the context of allergy, hypersensitivity refers to a
condition in a previously exposed person in which tissue
inflammation results from an immune reaction upon re-
exposure to an allergen sensitizer.
What is the "late phase" of an allergic reaction?
About 50% of the time, the allergic reaction progresses
into a "late phase." This "late phase" occurs about 4 to 6
hours after the exposure. In the late phase reaction,
tissues become red and swollen due to the arrival of other
cells to the area, including the eosinophils, neutrophils,
and lymphocytes. Cytokines that are released by the mast
cells and basophils act as tiny messengers to call these
other cells to the area of inflammation. Additional
cytokines are released by the TH2 lymphocytes and they
attract even more of these cells of inflammation.
The eosinophils appear to be particularly troublesome
cells of inflammation. Eosinophils evolved to defend the
body against parasites, much like IgE. Nevertheless, they
are often present in great numbers in the blood of people
with allergies. When they arrive at the site of the
allergic reaction, they release chemicals that cause damage
to the tissues and continue to promote the inflammation.
Repeated episodes of this "late phase" reaction contribute
to chronic allergic symptoms and make the tissues even more
sensitive to subsequent exposure!
Next: What are the consequences of the allergic cascade? »
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