27 
minimal lethal dose may be defined as that quantity of toxine 
i I which will surely kill eveiy guinea pig weighing 250 grams in the course 
\ I of four days or, at the very latest, five days. As Ehrlich has pointed 
j ^ out, such a quantih" may kill some of the animals sooner, that is, within 
I I thirty-six to forty-eight hours. A quantity of diphtheria poison that 
I will kill every guinea pig, without exception, acutely, within thirty- 
I six to forty-eight hours, contains more than the minimal lethal dose. 
* The L+ dose is a much more definite and constant factor than the 
MED. It does not show the irregularities or present the ditficulties 
met with in determining the absolute toxicity of the poison. 
In his earlier work Ehrlich looked upon the diphtheria toxine and 
antitoxin as simple substances neutralizing each other as an alkali does 
an acid. That the relation between these two substances is much more 
complex was at once evident as sooiT as he worked out the complex 
I nature of the diphtheria poison. 
! . The best method of studying the relations between toxine and anti- 
toxin is bv mixing these two substances together in varving amounts 
and studying the eftects of partial saturation or neutralization. As 
the immunity unit is supposed to contain 200 ‘‘combining units,” the 
most valuable information is obtained bv adding one two-hundredths of 
the immunit}" unit to a given quantity of toxine, using either the or 
L+ dose of toxine. These mixtures, inoculated into a series of guinea 
pigs of standard weight and under standard conditions, will give cer- 
tain definite results. 
B}" mixing with the L® dose of the diphtheria toxine such fractional 
amounts of antitoxin we have results which ma}" be summarized as 
follows: 
At first these mixtures ma}- show no diminution in toxicity, despite 
the addition of considerable (sa}", fort}- two-hundredths) of the immunity 
unit. This zone represents the prototoxoids. Then comes a time when 
for each addition of one two-hundredth of a unit of antitoxin the poison 
mav lose one minimal lethal dose. This is the zone of toxins {sy7\- 
toxins). Then follows the zone in which the mixture fails to produce 
acute death, but may cause local edema and later paral 3 "sis. This is 
the zone of toxones^ first called epitoxones b}^ Ehrlich. 
Later, von Dungern^^ showed the presence of epitoxonoids which 
have the same combining affinity for the antitoxin, but show no poison- 
ous properties.- The epitoxonoids of von Dungern explain the power 
of neutral mixtures of toxine and antitoxin to produce immunity 
when injected into susceptible animals. 
The results of the partial saturation or neutralization tests are con- 
veniently studied b}^ plotting curves, called by Ehrlich “spectra.”’ 
Several such spectra are shown in fig. 2. 
^ Beitragzurkenntnissder bindungsverhaltnisse bei der vereinigung von diphtherie- 
gift iind antiserum. Dent. med. woch., v. 30 (8-9), 1904, pp. 275-277, 310-312. 
