THEORIES OF IMMUNITY 413 



convenient to marshal together these various terms in a table as 

 follows : 



Immunity in manj = a c n dition of protection or insusceptibility to certain 



1. Natural immunity = constitutional protection produced by alexines. 



Acquired naturally (involuntary) produced by anti- 



toxins formed* by an attack of the disease. 

 Acquired artificially (voluntary) = 

 (a) Active immunity, produced by direct inocula- 

 tion of the weakened bacteria or weakened 

 toxins of the disease, e.g. vaccination, or 

 Pasteur's treatment of rabies, or Haffkine's 



. , . ., inoculation for cholera. 



Acquired immunity^ 



(ft) pa ^ immunity> produce d by inoculation, 

 not of the disease or of its toxins, but of the 

 antitoxins produced in the body of an animal 

 suffering from the specific disease. These 

 antitoxins combine in some way with the 

 toxins, and so avert their harmful effects. 

 An example of passive immunity occurs in 

 diphtheria antitoxin. 



Theories of Immunity 



We may now consider shortly how these new facts were received, 

 and what theories of explanation were put forward to explain con- 

 tinued insusceptibility to disease. It had, of course, been known for 

 a long time that one attack of small-pox, for example, in some 

 degree protected the individual from a subsequent attack of the 

 same disease. To that experience it was now necessary to add a 

 large mass of experimental evidence with regard to toxins and 

 antitoxins. The chief theories of immunity which have been pro- 

 pounded are as follows : 



1. The Exhaustion Theory. The supporters of this view argued 

 , that bacteria of disease circulating in the body exhausted the body of 



the supply of some pabulum or condition necessary for the growth 

 and development of their own species (Pasteur). 



2. The Retention Theory. This theory, on the contrary, was based 

 upon the view that there were certain products of micro-organisms 

 of disease retained in the body after an attack which acted antagon- 

 istically to the further growth in the body of that same species, as 

 occurs in a test-tube culture. 



3. The Acquired Tolerance Theory. Some have advanced the 

 theory that, after a certain time, the human tissues acquired such 

 a degree of tolerance to the specific bacteria or their specific products, 

 that no result followed their action in the body. The tissues became 

 acclimatised to the disease. 



4. The Phagocyte Theory. This theory, which gained so many 

 adherents when first promulgated by Metchnikoff, attributes to 



