Antivenom.
As a post script to her article on
venom, Bianca op den Brouw
looks at how antivenom works,
and why is it so difficult to achieve
a ‘broad-spectrum’ antivenom?
Antivenom is produced by injecting a host animal,
such as a horse, with increasing amounts of a given
venom (or venoms) over time. The venom toxins are
present in quantities small enough that they do not
cause too much harm, though they are still detected
by the animal’s body as foreign molecules, or antigens.
This triggers a response in the immune system of the
host animal, inducing the production of antibodies.
Antibodies are proteins which are able to recognise an
antigenic ‘epitope’: a face, of sorts - a molecular site
unique to that antigen. Each type of antibody is essen-
tially only able to recognise one face - one epitope - and
therefore an antibody is said to be specific to an antigen.
This is why multiple vaccines are required for immunisa-
tion against the influenza virus – each mutated form of
the virus has a different epitope. The ability to recognise
epitopes with such high specificity is what enables the
immune system to differentiate foreign molecules from
normal body molecules. Once an epitope is recognised,
the specific antibody will then bind to the antigenic toxin
which, depending on the toxin structure, either neutral-
ises its harmful activity on the body, or acts as a flag to
immune cells to locate and destroy it.
As the bioactivity and structure of the numerous toxins
composing venom differ, each toxin type has different
antigenic properties and a different epitope. Therefore,
each toxin type stimulates the production of toxin-
specific antibodies by the host animal’s immune system.
This cocktail of antibodies is then extracted and purified
from the blood of the host animal, and the resulting
mixture is used as an antivenom. The composition of the
antibodies comprising an antivenom is therefore specific
to the venom or venoms used during the antivenom
production. This is why different antivenoms are needed
for different species of snake. This is also why variation in
venom composition within a single snake species can
seriously compromise the effectiveness of an antivenom.
Image by Chuck Rausin.
Further, as the immune response does not discriminate
between antigens, antibodies are produced for all
components of venom, regardless of how much harm
they are capable of inducing. Therefore, if too many
different venoms are used to produce an antivenom, the
essential antibodies are diluted by the presence of
non-essential antibodies, thus requiring substantially
more antivenom to achieve the same effect.
Left: an Irula tribes-
man milks a snake
for antivenom
production at the
Madras Croc Bank.
The Irula Co-op will
feature in an
upcoming article.
Image courtesy
Madras Croc Bank.
Below: variation in
venom composition
can seriously com-
promise the
effectiveness of
antivenom. Image
by Michael Cermak.