IMMUNITY AND SUSCEPTIBILITY 687 



As mentioned above, it may be that the true explanation of the 

 phenomena of predisposition is found in anaphylaxis or the sensitiza- 

 tion to various proteins of microorganisms. Further work is necessary 

 along these lines. 



IMMUNITY 



Immunity and susceptibility to disease are always relative and 

 never absolute; that is, it is always possible to produce some sort of an 

 infection in a supposedly immune animal by modifying the conditions 

 under which the animal is accustomed to live. For example, the 

 chicken is immune to tetanus but by keeping this animal for some time 

 at a temperature higher than its normal it may be infected. The cow 

 cannot ordinarily be infected with typhoid but when large numbers of 

 the B. typhosus are injected under the skin an abscess may be produced. 

 These and many other examples might be mentioned. Our standard 

 of immunity in a particular animal is based upon the conditions as they 

 exist naturally and on the average resistance of animals of the same 

 species. 



Immunity to disease may be of two kinds, natural and acquired. 

 Natural immunity is that resistance which is possessed normally by an 

 individual. Acquired immunity is that resistance which is acquired 

 by having an infection, or by being vaccinated, or immunized against 

 an infection with the specific etiological microorganism or its antiserum. 



NATURAL IMMUNITY AND SUSCEPTIBILITY. Attention should be 

 directed to certain forms of natural immunity and susceptibility. 



Racial Immunity and Susceptibility. It is a familiar fact that certain 

 species of animals and certain races of man differ in their resistance and 

 their susceptibility to infectious diseases. As examples of racial im- 

 munity among animals the native cattle of Austria and Hungary and of 

 Japan which are relatively immune to bovine tuberculosis, a disease 

 which causes great loss among other breeds, may be mentioned. Again, 

 the sheep of Algeria are relatively immune to anthrax while all other 

 sheep are extremely susceptible. Field mice are immune to glanders 

 while the common house mouse is susceptible. The negro is more re- 

 sistant to the infectious microbic agent of yellow fever than other races, 

 but is without doubt more susceptible to tuberculosis. The Japanese 

 are said to be more resistant to scarlet fever than other races. The 



