i8 THE PRINCIPLES OF IMMUNOLOGY 



ceeded by one of depression. The stimulation of cells in the produc- 

 tion of antitoxic and anti-bacterial substances will be discussed sub- 

 sequently. 



Classification of Natural Immunity Species Immunity. As has 

 been indicated above, natural immunity may be found in species, 

 races, families, or individuals. It is profitable to emphasize again 

 that what we speak of as species immunity expresses a difference in sus- 

 ceptibility exhibited by certain species as contrasted with others. 

 Whereas man is susceptible to such diseases as syphilis, gonorrhea, 

 cholera, and diphtheria, numerous other species are resistant to these 

 diseases. It is possible to inoculate syphilis in higher apes, in the 

 rabbit, possibly in the guinea-pig and other animals, but even suc- 

 cessful inoculation shows a greater degree of resistance than is pos- 

 sessed by man. Conversely, man is not susceptible to hog-cholera, 

 chicken-cholera, rat-typhoid, and certain other diseases. Man is 

 susceptible to the bacillus of human tuberculosis, but less so to 

 that of bovine tuberculosis, still less to that of avian tuberculosis, 

 and not at all to that of fish tuberculosis. In fact, with the exception 

 of the rabbit, fish tuberculosis is not transferable to any of the warm- 

 blooded animals. Fish are not susceptible to human tuberculosis. 

 Practically all animals are susceptible to snake venoms except the 

 hog. Man is highly susceptible to pneumococcus and to bacillus 

 pestis, but fowl are resistant to both these organisms. Metchnikoff 

 showed that certain species of insects are susceptible to diphtheria 

 toxin whilst others are not. Man is susceptible to trypanosoma gam- 

 biense, but is resistant to trypanosoma naganae. In some instances 

 these variations in susceptibility and resistance depend upon the 

 environment. For example, frogs kept in low temperature are not 

 susceptible to anthrax, but if kept in a temperature of 35 C. they 

 succumb to the disease. Similarly it was found that if lizards are 

 kept at 16 C. they could not be infected with plague, but at a higher 

 temperature were susceptible. The work of Pasteur with anthrax 

 in fowl is a classical experiment. He found that if he kept fowl at 

 low temperatures they became susceptible to anthrax because of the 

 decrease of body temperature ; but if they were allowed to maintain 

 their normally high body temperature they were resistant. The 

 temperature of most of the lower mammalia is higher than that of 

 man, but the difference is not sufficiently great to explain all the varia- 

 tions in susceptibility and resistance. 



Racial immunity probably exists but cannot be so conclusively 

 proven in man as is true of species immunity. It is generally be- 

 lieved that Caucasians are less susceptible to tuberculosis than 

 negroes. That this is an inherent character of the race appears to be 

 somewhat doubtful. Difference in hygienic conditions and in de- 

 gree of exposure to the disease may account for much that appears 

 to be racial susceptibility. It is possible that the superior hygienic 

 conditions of whites in northern latitudes explains this difference. 

 It is also possible that having been the victims of tuberculosis for 



