27 
The minimal lethal dose may be defined as that quantity of toxine 
which will surel}^ kill eveiy guinea pig weighing 250 grams in the course 
of four days or, at the very latest, five days. As Ehrlich has pointed 
out, such a quantity may kill some of the animals sooner, that is, within 
thirty-six to forty-eight hours. A quantity of diphtheria poison that 
will kill every guinea pig, without exception, acutely, within thirty- 
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 
MLD. It does not show the irregularities or present the difficulties 
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 soon as he worked out the complex 
nature of the diphtheria poison. 
The best method of studying the relations between toxine and anti- 
toxin is by mixing these two substances together in vaiying amounts 
and studying the effects of partial saturation or neutralization. As 
the immunity unit is supposed to contain 200 ‘‘combining units,” the 
most valuable information is obtained by adding one two-hundredths of 
the imimmit}" unit to a given quantity of toxine, using either the L*' 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. 
By mixing with the L'' dose of the diphtheria toxine such fractional 
amounts of antitoxin we have results which may be summarized as 
follows: 
At first these mixtures ma}" show no diminution in toxicit}^, despite 
the addition of considerable (say, forty two-hundredths) of the immunit}^ 
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 
may lose one minimal lethal dose. This is the zone of toxins {stpi- 
toxins). Then follows the zone in which the mixture fails to produce 
acute death, but may cause local edema and later paralysis. This is 
the zone of toxones, first called epitoxones by 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 by plotting curves, called by Ehrlich “spectra.” 
Several such spectra are shown in fig. 2. 
«Beitragzurkenntnissder bindungsverhaltnisse bei der vereinigung von diphtlierie- 
gi ft und antiserum. Dent. ined. woch., v. 80 (8-9), 1904, pp. 275-277, 310-312. 
