IMMUNITY. 165 



tetanus, and mice and rats bear large doses of the diphtheria toxin 

 without ill effect. On the other hand, many animals that are sus- 

 ceptible to the toxins cannot be infected with the bacteria which pro- 

 duce these toxins. It will be seen from this that there may be both 

 bacterial and toxin immunity. 



We may discuss the subject of immunity in detail under the fol- 

 lowing heads : (1) Natural immunity, or that possessed by certain 

 species or races of animals at all times against certain diseases ; (2) 

 inherited immunity, which may be transmitted from the mother to 

 the fetus through the placental circulation, or from the mother to the 

 child through the milk ; (3) acquired immunity, or that which is 

 secured by one attack of the disease, by vaccination, or by repeated 

 treatments with sterilized or unsterilized cultures. Acquired im- 

 munity may be either active or passive. By an active immunity we 

 mean that form which is induced by the direct treatment of the ani- 

 mal with filtered or unfiltered cultures, and in which case the anti- 

 toxic or anti-bacterial substance is produced in the body of the animal 

 thus treated. Passive immunity, on the other hand, may be secured 

 by injecting the blood serum of an animal actively immunized into a 

 second animal. As an illustration of active and passive immunity, we 

 may refer to the method by which diphtheria antitoxin is prepared. 

 The horse is treated with successive, non-fatal, gradually increased 

 doses of diphtheria toxin until this animal furnishes a serum which 

 contains a large amount of diphtheria antitoxin. The horse's serum 

 when injected into the child sick with diphtheria supplies the anti- 

 toxin, which combines with the toxin, and thus protects the tissues 

 of the child against the injury that would otherwise be inflicted on 

 the cellular elements by the noxious agent. In this illustration an 

 active immunity is induced in the horse, and as a result of the es- 

 tablishment of this condition certain cells within the animal are 

 stimulated to a form of activity by which the antitoxin is generated. 

 In other words,''the horse treated with successive doses of the diph- 

 theria toxin comes to possess an active immunity. The horse's 

 serum containing the antitoxin is injected into the child, which for 

 the time being becomes physiologically a part of the horse, and pos- 

 sesses only a passive immunity. 



We will now proceed to discuss in more detail the subject of nat- 

 ural immunity. It is a fact of common observation as well as of ex- 

 perimental demonstration that the lower animals are wholly immune 

 to certain bacterial infections to which man is markedly susceptible. 

 For instance, among men typhoid fever is one of the grave diseases, 

 causing great morbidity, and increasing to a considerable extent the 

 mortality lists ; while among the lower animals this disease does not 

 occur naturally, nor has anyone as yet been able to induce it by in- 

 oculation with the bacillus. It is true that many of the lower 

 animals are susceptible to the typhoid toxin, but a true typhoidal in- 



