The interaction of the antibody with an antigen causes a change
in shape of the antibody. The variable regions shown as black
or gray in the graphic on the left are the areas of the receptor
site for the antigen.
This in turn may cause the exposure of another site which
then is responsible for the various reactions elicited by the
antibody to destroy the foreign substance. The interaction of
antibodies and antigens may produce a network type complex.
Rini, et al.have compared the structures of a Fab fragment
of a monoclonal antibody to influenza virus hemagglutinin (HA)
in both ligand-bound and unliganded forms.
At the left is displayed the 3-D structure of the V-A and
V-C portions of the Fab complexed with the heptapeptide antigen
from HA. Examination of the antigen binding pocket reveals that
a pronounced conformational change has occurred upon antigen
binding. The pocket is deformed by the antigen, closing around
it. This is mostly caused by a shift in the orientation of V-A,
represented by the gray and red. The red is the orientation of
Asp99 and Asn100.
Immunoglobin - Antigen - Chime
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the Immune System - NIH - many pages
The understanding of immunochemistry led to the use of vaccinations
as protection against various epidemic producing diseases such
as small pox, diphtheria, polio, and typhus. A vaccination works
by injecting a small amount of weakened disease producing virus
into the body which elicits the production of antibodies which
eventually destroy the foreign substance. Subsequent invasions
of the body by this same virus are met and destroyed by the antibodies
formed at the time of vaccination.
Credit: Rini, J.M., Schultze-Gahmen, U., and Wilson, I.A.
Structural Evidence for Induced Fit as a Mechanism for
Antibody-Antigen Recognition. Science 255: 959-965.