172 THE VARIOUS TYPES OF IMMUNITY 



may be due to the presence or production of anti-aggressins. This im- 

 munity would seem to depend not upon the bactericidal properties of 

 the serum or leukocytes nor upon the antitoxins, but on the presence of 

 substances that prevent the microorganisms from exercising their special 

 aggressive forces. 



9. Finally, an immunity may exist because the parasite or other for- 

 eign cell does not obtain suitable nutrition in a host and thus fails to grow. 

 This condition of^athrepsia, is responsible for what has been called athreptic 

 immunity. It has been more recently studied by Ehrlich, who found 

 that upon transferring mouse cancer to the rat, the tumor grew for a 

 short time only, or presumably until the special nutriment carried over 

 with the tumor was consumed. While there is no experimental basis 

 for assuming that a similar condition may be present in bacterial life, 

 yet such a cause may be operative and should be kept in mind. 



ACQUIRED IMMUNITY 



Acquired immunity occurs in two distinct forms: (1) Active and (2) 

 Passive. A mixed form may exist, brought about by a combination 

 of factors necessary for the development of the other two. 



Active Acquired Immunity. Active acquired immunity is that form of 

 resistance to infection brought about by the activity of the cells of a person 

 or animal as a result of having had the actual disease in question, or as a 

 result of artificial inoculation with a modified or attenuated form of the 

 causitive microparasite. 



The essential feature of this immunity is that the cells and tissues 

 of persons or animals should, by their own efforts, and as a result of 

 their own active struggle against the action of a microparasite or its 

 products, overcome these and become less susceptible to them than they 

 were before. 



This form of immunity is gained, therefore, only as the result of an 

 active struggle between body-cells and infecting agent, and this battle 

 may be of any degree of severity, ranging from an attack of the disease 

 itself that may threaten life, down to the most transitory and trivial re- 

 action due to the injection of a minute dose of a mild vaccine. 



Active acquired immunity may be gained (1) By accidental infection, 

 which is the most familiar form of acquired immunity, and follows an 

 attack of an infectious disease, such as scarlet fever, measles, varicella, 

 variola, or typhus fever; (2) by inducing an attack of the disease by arti- 

 ficial inoculation. This latter method of producing an active acquired 



