72 ESSENTIALS OF BACTERIOLOGY. 



that the albumin part of the toxin is furnished by the blood or 

 albuminous media in which it is formed. The term toxin is to 

 be preferred in speaking of bacterial poisons. 



Toxins may be of two sorts : (a) Chiefly within the bodies of 

 the bacteria, so that they are set free by the disintegration of 

 the organisms. This group comprises most of the pathogenic 

 bacteria and must be combatted by the use of antibacterial sera. 

 (b) The poisons seem to be excreted by the bacteria and are 

 found in the surrounding medium. Antitoxic sera are applica- 

 ble to this group, which includes the bacilli of diphtheria and 

 tetanus. Welch has suggested that even bacteria which do not 

 appear to form toxins in artificial cultures may do so in the 

 human body. In the effort to adapt themselves to their 

 environment and resist the hostile agencies of the body they 

 produce the poisons we call toxins. (For method of produc- 

 tion of an antitoxin, see article on Diphtheria.) 



CHAPTER XIV. 



IMMUNITY. 



Immunity, as distinguished from susceptibility, is merely a rela- 

 tive term, as no animal is absolutely immune under all condi- 

 tions. It is merely less susceptible, and some animals are by 

 nature or can by artificial means be rendered so slightly sus- 

 ceptible that to all practical purposes they are immune that is, 

 capable of resisting an attack of the particular disease against 

 which they are said to be immune. 



Natural Immunity. The goat and dog are considered 

 naturally immune to tuberculosis. Algerian sheep are resistant 

 to anthrax, other varieties are susceptible. 



The field mouse is susceptible to glanders, the white mouse 

 is ordinarily immune. House mice are susceptible to mouse 

 septicemia, field mice are immune. 



Acquired Immunity. Immunity can be acquired in many 

 ways. Active and passive immunity are varieties. 



Active immunity can be acquired from an attack of the disease; 



