170 
PRINCIPLES OF BACTERIOLOGY 
the patient should not be allowed to mingle with others 
until at least two cultures made from their throats have 
been found negative. 
The Schick test is based on the principle that persons 
whose natural content of antitoxin is low, i. e., insuffi¬ 
cient to protect them from diphtheria, can be identified 
by injecting into their skin a small amount of diphtheria 
toxin when in such persons a marked reaction will take 
place at the site of injection, while in persons whose con¬ 
tent of antitoxin is normal, no such reaction will take 
place. In practice the amount of diphtheria toxin in¬ 
jected is 1/50 of the amount sufficient to kill, in four days, 
a guinea pig weighing 250 grams. 
Prevention of diphtheria means isolation and quaran¬ 
tine of the patient and nurse, strictest disinfection of all 
utensils, bedding, etc., and injection of diphtheria anti¬ 
toxin to all those who had come in contact with the 
patient. Schick’s test will show who needs such injec¬ 
tions. 
When Schick’s test is positive, showing that the pa¬ 
tient does not possess the normal amount of diphtheria 
antitoxin, instead of giving the patients some anti¬ 
serum, today they are given an injection of the toxin 
and the antitoxin mixture, which is more efficient, be¬ 
cause the toxin (carefully neutralized by antitoxin) acts 
as a prolonged and slow T stimulant to the patient’s or¬ 
ganism to produce his own antitoxin. 
VI. Mechanism of Infection and Immunity 
Diphtheria bacillus, like the tetanus, produces an exo¬ 
toxin ; it was in connection with this exotoxin that most 
of the experimental work on immunity was done by Ehr¬ 
lich and his pupils. The antibody produced in infected 
animals is, of course, an antitoxin. The antitoxin is pro- 
