s for
antitoxin production, according to Ehrlich's theory, is afforded. The
theory also supplies the explanation of the power which an animal possesses
of producing various antitoxins, since this depends ultimately upon
susceptibility to toxic action. The explanation is thus carried back to the
complicated constitution of biogen molecules in various living cells of the
body. It may be added that in the case of all the other kinds of
anti-substances, which are produced by a corresponding reaction, we have
examples of the existence of traces of them in the blood serum under normal
conditions. We are, accordingly, justified in definitely concluding that
their appearance in large amount in the blood, as the result of active
immunization, represents an increased production of molecules which are
already present in the body, either in a free condition in its fluids or as
constituent elements of its cells.

[Sidenote: Anti-bacterial serum.]

In preparing anti-bacterial sera the lines of procedure correspond to those
followed in the case of antitoxins, but the bacteria themselves in the
living or dead condition or their maceration products are always used in
the injections. Sometimes dead bacteria, living virulent bacteria, and
living supervirulent bacteria, are used in succession, the object being to
arrive ultimately at a high dosage, though the details vary in different
instances. The serum of an animal thus actively immunized has powerful
protective properties towards another animal, the amount necessary for
protection being sometimes almost inconceivably small. As a rule it has no
action on the corresponding toxin, _i.e._ is not antitoxic. In addition to
the protective action, such a serum may possess activities which can be
demonstrated outside the body. Of these the most important are (a)
bacteriolytic or lysogenic action, (b) agglutinative action, and (c)
opsonic action.

[Sidenote: (a) Lysogenic action.]

The first of these, lysogenic or bacteriolytic action, consists in [v.03
p.0178] the production of a change in the corresponding bacterium whereby
it becomes granular, swells up and ultimately may undergo dissolution.
Pfeiffer was the first to show that this occurred when the bacterium was
injected into the peritoneal cavity of the animal immunized against it, and
also when a little of the serum of such an animal was injected with the
bacterium into the peritoneum of a fresh, _i.e._ non-immunized animal.
Metchniko