A _polyvalent_ serum, that is, one derived from an animal which has been
immunised by numerous strains of the organism derived from various
sources, is much more efficacious than when a single strain has been
used.
_Clinical Use of Serums._--Every precaution must be taken to prevent
organismal contamination of the serum or of the apparatus by means of
which it is injected. Syringes are so made that they can be sterilised
by boiling. The best situations for injection are under the skin of the
abdomen, the thorax, or the buttock, and the skin should be purified at
the seat of puncture. If the bulk of the full dose is large, it should
be divided and injected into different parts of the body, not more than
20 c.c. being injected at one place. The serum may be introduced
directly into a vein, or into the spinal canal, _e.g._ anti-tetanic
serum. The immunity produced by injections of antitoxic sera lasts only
for a comparatively short time, seldom longer than a few weeks.
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