INFECTION AND IMMUNITY. 401 



It is a common observation that certain human beings 

 and animals are more susceptible to the different forms 

 of infection than are others, and that some are apparently 

 not at all liable to particular diseases ; in other words, 

 they are naturally immune to the maladies. 



Again, it is often observed that an individual or 

 animal after having recovered from certain forms of 

 infection has thereby acquired protection against subse- 

 quent attacks of like character ; in other words, they 

 are said to have acquired immunity to this trouble. 



The problem involving the explanation of these in- 

 teresting observations has afforded material for reflection 

 and hjrpothesis for a long time, but it is only through 

 investigations that have been conducted during the past 

 few years that it has met with anything approaching 

 reasonable solution. 



Conspicuous among the observers who have endeav- 

 ored to explain the modus operandi of immunity may 

 be mentioned Chauveau, Pasteur, Metchnikoff, Buch- 

 ner, Fliigge, and his pupils (Smirnow, Sirotinin, 

 Bitter, Nuttall) Fodor, and Hankin, and in the follow- 

 ing pages we will present briefly the results of investi- 

 gations by these various authors. 



In 1880 Chauveau' suggested an explanation for the 

 phenomenon of immunity that has since been known 

 as the retention hypothesis. It is, in short, as fol- 

 lows : That the immunity commonly seen to exist in 

 animals that have passed through an attack of infection, 

 against a subsequent outbreak of the same malady, and 

 likewise the immunity that has been produced artificially 

 by vaccination, exists by virture of some bacterial pro- 

 duct that has been retained or deposited in the tissues of 



1 Comptes-rendus, etc., No. 91. July, 1880. 



