70 CLINICAL BACTERIOLOGY. 



After this, the amount of antitoxin in the blood gradually 

 diminishes. It now appears in the milk, in the urine, etc., 

 until finally it is wholly swept out of the body. The 

 rapidity with which this elimination of antitoxin takes place 

 is most variable, in accordance with the different conditions 

 present. It is the greater the larger the amount of immu- 

 nizing serum injected. In the case of diphtheria, the pro- 

 tection in human beings following the usual immunization 

 with 250 antitoxin normal units lasts about four weeks. 



Success in the preparation of antitoxic serum may be 

 hoped for only when the poison, the toxin of the respective 

 species of bacteria, is known, and can be prepared of suffi- 

 cient strength. It has thus far been possible to obtain a 

 high degree of toxin-immunity only in the case of diph- 

 theria, tetanus, botulism, snake-venom, ricin, and abrin. 



It has already been mentioned that the lysogenic and the 

 agglutinating substances of the serum are also present in 

 the blood of normal individuals. The same statement 

 applies also to the antitoxins. Attention has been called 

 by a number of observers to a certain neutralizing activity 

 on the part of the normal blood-serum of horses and of 

 human beings against the poison of diphtheria and other 

 similar poisons. The natural antitoxic pozver of the blood- 

 serum is, however, only slight. It by no means attains the 

 high degree of activity exhibited by the serum of animals 

 artificially made poison-proof. 



From the fact that immunity can be transmitted by 

 means of serum containing antitoxin, the doctrine that 

 protection against toxins is the cause of immunity has been 

 brought forward. Behring, and also Ehrlich, ascribe the 

 real cause of acquired immunity to the antitoxic property 

 of the blood. However attractive this theory may be, all 

 of the existing facts can not be brought in harmony with 

 it. Reference may be made to the instances of a want of 

 relation between the occurrence of antitoxin in the blood 

 (immunizing capability of the blood-serum) and the pres- 

 ence of immunity (p. 55). It was mentioned that the 

 occurrence of antitoxin in the blood of animals by no 

 means establishes necessarily a condition of immunity in 

 the latter, and that, under certain circumstances, the or- 

 ganism whose blood-serum possesses pronounced immu- 

 nizing capability, exhibits not only not an increased, but 

 even a diminished, degree of resistance to the bacterial 



