IMMUNITY AND SUSCEPTIBILITY. 83

amount of toxin endurance of a resistent animal be repre-
sented by x, and any addition to this as y. Then xy
would certainly be fatal. If the least quantity of anti-
toxin that will protect the animal be expressed by 2, then
xy + z is harmless. It is evident, however, that z does
not necessarily have any influence upon x^ but only need
neutralize y in order to save the animal, and therefore
it is obvious that the remaining x in such a mixture
could readily destroy another more susceptible animal
into which it might be injected.

I am of the opinion that the effect of the antitoxin
really partakes of the nature of chemic neutralization
from the following experiment: let a" represent the least
certainly fatal dose of diphtheria toxin for a guinea-pig,
and y the least quantity of antitoxin that will protect
against it; then

x + y is harmless. That

10 x + io.y is also harmless is known to every one
accustomed to test antitoxins. I have con-
tinued this and have found that
50 •*: + 50 y
100 x + 100 y are also harmless.

According to Buchner, the antitoxins differ from the
alexins in being new substances in the blood, in being
without germicidal or chemical neutralizing power against
the toxins, and in being stable compounds which can
resist heat to 75° C., can resist a reasonable amount of
exposure to light, and which are not altered by decompo-
sition of the substances containing them.

The antitoxins are specific for one poison only. Ehrlich
found that antiricin was powerless against abrin, and vice
versd. Diphtheria antitoxin is of no avail against tetanus,
and vice versd.

The immunity which the antitoxins produce is fuga-
cious, varying considerably according to the particular
substance employed. As a rule, it is limited to a few
months—at least in the case of such antitoxins as we can
produce experimentally.