IMMUNITY AND IMMUNIZING AGENTS. 115 



It has been known for a long time that when a number of individuals of 

 the same species are exposed to the same bacterial infection, some escape the 

 infection while others do not. That which prevented the development of the 

 bacterial disease or which neutralized the toxic products or which killed the 

 disease organisms thus preventing the disease manifestations, constitutes 

 immunity. 



It may be assumed that the members of the various subdivisions of the 

 animal kingdom have in the course of their phylogenetic or evolutional de- 

 velopment acquired certain properties of cells, cell-contents, tissues and 

 organs, which enable them to resist certain harmful bacterial invasions, as 

 well as the injurious effects of other noxious influences and substances. 

 For example, the typhoid fever germ is harmless to the oyster and other lower 

 as well as most higher animals, but it is very injurious to man. The vege- 

 table alkaloids are very toxic to man and most other vertebrates, whereas 

 they are harmless to the protozoa and other low forms of animal life. As is 

 known drug parasites feed with impunity upon the most potent vegetable 

 drugs. The carnivora are less liable to bacterial infection than the her- 

 bivora. Closely related species sometimes display remarkable immunity 

 differences. For example, field mice are very susceptible to glanders, whereas 

 the common house mouse is almost wholly immune. Jersey cows are less 

 liable to tuberculosis than Holsteins. The Yorkshire breed of swine is less 

 liable to the attacks of hog erysipelas than are other breeds. Man is 

 especially susceptible to malaria, cholera and typhoid fever. Man, cattle 

 and apes are very susceptible to infection by the tubercle bacillus, whereas 

 the wild carnivora are quite exempt. Anthrax may attack man, cattle, 

 sheep and guinea-pigs, whereas birds, rats, cats and dogs are free from such 

 attack. The Caucasian race is less liable to small-pox, tuberculosis and 

 syphilis than is the Negro race. On the other hand, the Negro is more 

 immune to yellow fever than is the white man. 



Immunity is, however, very largely relative. The wild carnivora are quite 

 free from disease whereas in prolonged captivity they may fall prey to 

 several diseases, notably tuberculosis. Toxic substances, noxious gases, 

 lack of food, poor food, cold, excessive heat, fatigue, over-exertion, inclement 

 weather, etc., are factors which may lessen the natural immunity to the 

 several infections to which the animal may be exposed. For example, no 

 race of mankind is possessed of absolute immunity to any human disease. 

 Such immunity differences as are observed are due to differences in the op- 

 portunities for infection, differences in habit, in occupation, etc. 



The modern explanations regarding the mechanism of immunity are 

 extremely interesting and a work on pharmaceutical bacteriology would 

 certainly be incomplete without a brief summary of the discoveries 

 to date. 



