ay occur in bacteria, cells, &c., or they
may occur free in a fluid. Anti-substances may be arranged, as has been
done by Ehrlich, into three main groups. In the first group, the
anti-substance simply combines with the antigen, without, so far as we
know, producing any change in it. The antitoxins are examples of this
variety. In the second group, the anti-substance, in addition to combining
with the antigen, produces some recognizable physical change in it; the
precipitins and agglutinins may be mentioned as examples. In the third
group, the anti-substance, after it has combined with the antigen, leads to
the union of a third body called _complement_ (_alexine_ or _cytase_ of
French writers), which is present in normal serum. As a result of the union
of the three substances, a dissolving or digestive action is often to be
observed. This is the mode of action of the anti-substances in the case of
a haemolytic or bacteriolytic serum. So far as bacterial immunity is
concerned, the anti-serum exerts its action either on the toxin or on the
bacterium itself; that is, its action is either antitoxic or
anti-bacterial. The properties of these two kinds of serum may now be
considered.

[Sidenote: Antitoxic serum.]

The term "antitoxic" signifies that serum has the power of neutralizing the
action of the toxin, as is shown by mixing them together outside the body
and then injecting them into an animal. The antitoxic serum when injected
previously to the toxin also confers immunity (passive) against it; when
injected after the toxin it has within certain limits a curative action,
though in this case its dose requires to be large. The antitoxic property
is developed in a susceptible animal by successive and gradually increasing
doses of the toxin. In the earlier experiments on smaller animals the
potency of the toxin was modified for the first injections, but in
preparing antitoxin for therapeutical purposes the toxin is used in its
unaltered condition, the horse being the animal usually employed. The
injections are made subcutaneously and afterwards intravenously; and, while
the dose must be gradually increased, care must be taken that this is not
done too quickly, otherwise the antitoxic power of the serum may fall and
the health of the animal suffer. The serum of the animal is tested from
time to time against a known amount of toxin, _i.e._ is standardized. The
unit of antitoxin in Ehrlich's new standard is the amount requisite