ANTITOXIC IMMUNITY. 133 



In contrast to infections of the type referred to 

 above are others in which the symptoms are pro- 

 duced by soluble toxins, the ectotoxins, which are 

 secreted by the micro-organisms. 



The symptoms which are so characteristic of 

 tetanus are produced, not by contact of the bacteria 

 with the nervous system, but rather through the 

 specific soluble toxin which is secreted by the bacilli 

 in the wound where they reside. This poison, or 

 toxin, is carried from the wound to the nervous 

 system through the lymphatic or blood circulation, 

 the bacterium itself not being transported. There- 

 fore, although tetanus is a bacterial disease, it is 

 at the same time and in a peculiar sense a toxic 

 disease. The serum of an animal which has 

 acquired immunity to diphtheria or tetanus neu- 

 tralizes the corresponding soluble toxin, but does 

 not necessarily injure the micro-organism itself. 

 That is to say, the immunity is antitoxic. 



Experience has shown that this distinction 

 between antibacterial and antitoxic immunity is 

 an important one, and the differentiation is very 

 sharp in some instances, particularly in acquired 

 immunity. In many examples of natural immun- 

 ity, the resistance cannot be attributed so specific- 

 ally to antibacterial or antitoxic serum properties. 

 This is referred to later. 



In the types of immunity referred to, the fac- 

 tors on which the resistance appears to depend, 

 i. e., the gennicidal or antitoxic action of the 

 serum or the germicidal power of the leucocytes, 

 are susceptible to experimental demonstration. We 

 are familiar with another type of resistance, how- 

 ever, which finds no expression in the form of 

 antitoxins or other antibodies. This relates par- 



