nt of toxin-immunity the doses, small at first, are gradually
increased in successive inoculations; or, as in the case of very active
toxins, the initial injections are made with toxin modified by heat or by
the addition of various chemical substances. Immunity of the same nature
can be acquired in the same way against snake and scorpion poisons, and
against certain vegetable toxins, _e.g._ ricin, abrin, &c.

In order that the immunity may reach a high degree, either the bacterium in
a very virulent state or a large dose of toxin must ultimately be used in
the injections. In such cases the immunity is, to speak generally,
specific, _i.e._ applies only to the bacterium or toxin used in its
production. A certain degree of non-specific immunity or increased tissue
resistance may be produced locally, _e.g._ in the peritoneum, by injections
of non-pathogenic organisms, peptone, nucleic acid and various other
substances. In these cases the immunity is without specific character, and
cannot be transferred to another animal. Lastly, in a few instances one
organism has an antagonistic action to another; for example, the products
of _B. pyocyaneus_ have a certain protective action against _B. anthracis_.
This method has, however, not yielded any important practical application.

2. _Passive Immunity: Anti-sera._--The development of active immunity by
the above methods is essentially the result of a reactive process on the
part of the cells of the body, though as yet we know little of its real
nature. It is, however, also accompanied by the appearance of certain
bodies in the blood serum of the animal treated, to which the name of
_anti-substances_ is given, and these have been the subject of extensive
study. It is by means of them that immunity (passive) can be transferred to
a fresh animal. The development of anti-substances is, however, not
peculiar to bacteria, but occurs also when alien cells of various kinds,
proteins, ferments, &c., are injected. In fact, organic molecules can be
divided into two classes according as they give rise to anti-substances or
fail to do so. Amongst the latter, the vegetable poisons of known
constitution, alkaloids, glucosides, &c., are to be placed. The molecules
which lead to the production of anti-substances are usually known as
antigens, and each antigen has a specific combining affinity for its
corresponding anti-substance, fitting it as a lock does a key. The
antigens, as already indicated, m