234 GENERAL BIOLOGY OF MICRO-ORGANISMS 



Theories of Immunity. — Early theories of immunity were 

 based upon meager observations. The idea that an attack of a 

 disease left behind in the body something which prevented the 

 subsequent entrance of that disease was formulated by Chauveau 

 in 1880 as the so-called retention hypothesis. In the same year 

 Pasteur expressed the idea that an attack of a disease removed 

 something from the body and so exhausted the soil as far as that 

 particular disease was concerned. Neither of these ideas was 

 new at that time, and neither of them pretended to any very 

 definite or specific application to phenomena observed in immu- 

 ity, but only to the general phenomenon of immunity itself. 

 The discovery of phagocytosis by Metchnikoff in 1884 was the 

 first observation of a definite phenomenon which appeared to 

 explain the facts of immunity'. The phagocytic theory, which 

 grew out of this observation, was an attempt to ascribe immunity 

 in general to this one phenomenon of phagocytosis. With the 

 observation of the bactericidal substances in solution in the blood 

 plasma by Nuttall and by Buchner, of the antitoxins by von 

 •Behring and the bacteriolysins by Pfieffer, there developed at- 

 tempts to ascribe all the observed facts of immunity to these 

 factors, resulting in the alexin theory and the antitoxin theory 

 of immunity. More intimate study of the dissolved immune 

 bodies lead to the formulation of a hypothesis to explain their 

 formation, composition and action, the side-chain theory of Ehr- 

 lich, which has been of great value as a working hypothesis and 

 as a central conception about which to arrange the observed facts 

 relating to these dissolved substances. The elementary concepts 

 of this theory have been given in the preceding chapter. 



In brief, Ehrhch pictures the living cell as a chemical unit 

 possessing numerous and varied combining groups or side-chains 

 capable of uniting with substances in contact with the cell. The 

 toxin molecule is conceived as a substance containing at least 

 two distinct chemical groups, one which serves for attachment 

 to the side-chain of the cell and the other serving tobear the poison- 

 ous properties. The union of the toxin with the cell results in 



