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P A R T 3
Drugs acting on the immune system
form an antigen–antibody complex. This new structure
reveals a new receptor site on the antibody that activates
a series of plasma proteins in the body called comple
ment proteins.
Complement proteins
Complement proteins
react in a cascade fashion to
form a ring around the antigen–antibody complex. The
complement can destroy the antigen by puncturing its
membrane, allowing an osmotic inflow of fluid that
causes the cell to burst. They also induce chemotaxis
(attraction of phagocytic cells to the area), increase the
activity of phagocytes and release histamine. Hista
mine release causes vasodilation, which increases blood
flow to the area and brings in all of the components of
the inflammatory reaction to destroy the antigen. The
antigen–antibody–complement complex precipitates out
of the circulatory system and deposits in various sites,
including end arteries in joints, the eyes, the kidneys
and the skin. The signs and symptoms of the inflamma
tory response can be seen where the antigen–antibody
complexes are deposited. Chickenpox eruptions are an
example of an antigen–antibody–complement complex
that deposits in the skin and causes a local inflammatory
reaction.
Antibody formation
The initial formation of antibodies, or primary response,
takes several days. Once activated, the B cells form
memory cells that will produce antibodies for immedi
ate release in the future if the antigen is encountered.
The antibodies are released in the form of immuno
globulins. Five different types of immunoglobulins have
been identified:
• The first immunoglobulin released is M (IgM), which
contains the antibodies produced at the first exposure
to the antigen.
• IgG, another form of immunoglobulin, contains
antibodies made by the memory cells that circulate
and enter the tissue; most of the immunoglobulin
found in the serum is IgG.
• IgA is found in tears, saliva, sweat, mucus and bile.
It is secreted by plasma cells in the GI and respiratory
tracts and in epithelial cells. These antibodies react
with specific pathogens that are encountered in
exposed areas of the body.
A
C
B
D
Antigen B cell
Plasma cell
(IgM)
Memory cell
(IgG)
Antibodies
Ag-Ab complex
Activation of complement
Pain
Antigen destroyed
Vasodilation
↑
Capillary
permeability
Histamine
release
Kinin
activation
Chemotaxis Phagocytosis Lysis
Helper T cells
Suppressor T cells
Antigen
FIGURE 15.4
The humoral immune
response.
A.
A B cell reacts with a
specific antigen to form plasma cells
and memory cells, which produce
antibodies.
B.
Circulating antibodies
react with the antigen to form an
antigen–antibody (Ag-Ab) complex.
This process is facilitated by helper
T cells and suppressed by suppressor
T cells.
C.
The antigen–antibody
complex activates circulating
complement, which facilitates
aggressive inflammatory reactions.
D.
This process destroys the antigen.
