84	PA THOGENIC BA CFERIA.
A new principle discovered by Pfeiffer, and bearing
directly upon the theories of immunity, is that the se-
rum of animals immunized to certain diseases (cholera
and typhoid) contains a germicidal substance. Metchni-
koff has tried to show that the action of this body depends
upon solution of the leucocytes, but Pfeiffer has disproved
this by showing that the liquor puris from abscesses oc-
curring in the experiment-animals did not contain the
active substance.1
The work of van de Velde2 is very interesting. An
animal immunized by progressively increasing doses of
strong filtered toxin produced a serum possessed of pow-
erful anti-infectious and antitoxic powers; one immu-
nized by the introduction into its body of the washed,
precipitated bodies of diphtheria bacilli collected by ni-
tration furnished a serum of appreciable anti-infectious,
but no antitoxic properties; one immunized by the use
of bacillus cultures developed antitoxic and anti-infectious
serum identical with the first described; one immunized
to weak toxin furnished serum of considerable anti-infec-
tious, but slight antitoxic power, and still another ani-
mal that received toxin that had been heated developed
neither anti-infectious nor antitoxic serum.
Seeing that the serums commercially manufactured are
made by the use of strong filtered toxin, van de Velde
examined a number of samples purchased in the market,
and found that they were all possessed of both antitoxic
and anti-infectious properties. It is important to remem-
ber the presence of both of these properties in the serum,
as the successful use of the agent for immunizing depends
upon the presence of the one, and the use in treatment
upon the presence of the other.
Immunity and antitoxins stand in unknown relation-
ship to one another. That an animal has considerable
antitoxin in its blood is no guarantee that it is immune.
I have seen a horse in each c.cm. of whose blood there
1	Centralbl. f. Bakt. u. Parasitenk., Bd. xix., Nos. 14 and 15.
2	Ibid., Nov. 24, 1897, Bd. xxii., Nos. 18 and 19.