by a process comparable to narcotisation, and render them an easy prey
for the phagocytes.

_Artificial or Passive Immunity._--A form of immunity can be induced by
the introduction of protective substances obtained from an animal which
has been actively immunised. The process by which passive immunity is
acquired depends upon the fact that as a result of the reaction between
the specific virus of a particular disease (the _antigen_) and the
tissues of the animal attacked, certain substances--_antibodies_--are
produced, which when transferred to the body of a susceptible animal
protect it against that disease. The most important of these antibodies
are the _antitoxins_. From the study of the processes by which immunity
is secured against the effects of bacterial action the serum and vaccine
methods of treating certain infective diseases have been evolved. The
_serum treatment_ is designed to furnish the patient with a sufficiency
of antibodies to neutralise the infection. The anti-diphtheritic and the
anti-tetanic act by neutralising the specific toxins of the
disease--_antitoxic serums_; the anti-streptcoccic and the serum for
anthrax act upon the bacteria--_anti-bacterial serums_.

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