IMMUNITY AND ANTITOXIN'S 247 



and the antitoxins : when the toxins are in the ascendency 

 we get an increase of the disease; when the antitoxins are 

 in the ascendency we get a diminution of disease. If the 

 toxins triumph, the result is death; if the antitoxins and 

 resistance of the tissues triumph, the result is recovery. 



We may now consider shortly how these new facts were 

 received and what theories of explanation were put forward 

 to explain continued insusceptibility to disease. It had of 

 course been known for a long time past that one attack of 

 small-pox, for example, in some degree protected the in- 

 dividual 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 anti- 

 toxins. The theories of immunity were as follows: 



1. The Exhaustion Theory. The supporters of this idea 

 argued that bacteria of disease circulating in the body ex- 

 hausted the body of the supply of some substance or condi- 

 tion necessary for the growth and development of their own 

 species. 



2. The Retention Theory. It was surmised that there were 

 certain products of micro-organisms of disease retained in 

 the body after an attack which acted antagonistically to the 

 further growth in the body of that same species. 



3. The Acquired Tolerance Theory. Some have advanced 

 the theory that, after a certain time, the human tissues ac- 

 quired such a degree of tolerance to the specific bacteria or 

 their specific products that no result followed their action in 

 the body. The tissues become acclimatised to the disease. 



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

 many adherents when first promulgated by Metschnikoff, 

 attributes to certain cells in the tissues the powers of 

 '* scavenging," overtaking germs of disease, and absorbing 

 them into their own protoplasm. This, indeed, may be 

 actually witnessed, and had been observed before the time 

 of Metschnikoff. But it was he who applied it to disease. 



