174 IMMUNITY. 



the development of the first symptoms. Soon after the man has 

 been bitten he is, according to the Pasteur method, treated succes- 

 sively with dried portions of spinal cords taken from animals which 

 have died from experimental hydrophobia. These cords probably 

 contain the toxin of the disease, or the modified microorganism, or 

 both, and those which have been dried longest contain these in their 

 least active form. By successive inoculations with emulsions of 

 these cords immunity to the hydrophobia virus may be secured be- 

 fore the period necessary for the development of the germ introduced 

 with the bite of the dog has passed. 



For some years Emmerich has worked on a theory of immunity, 

 which, if it should prove to be correct, gives a simple explanation of 

 many phenomena which have puzzled bacteriologists. JRecently this 

 theory has been elaborated in two papers published by Emmerich and 

 Low,^ and its important points may be stated as follows : Many 

 bacteria, and among these some of the most important pathogenic 

 ones, produce, both m miro and in vivo, enzymes which are capable 

 of digesting the organism which produces them, and sometimes other 

 bacteria as well. In proof of this statement they bring forward the 

 following observation : If a culture of the bacillus pyocyaneus be 

 allowed to stand at 37° there forms after three days on the surface a 

 thick scum consisting of bacilli. If the tube be thoroughly shaken 

 some of the bacteria subside, and after three days longer a new scum 

 forms on the surface. The process of shaking and allowing to grow 

 is repeated six or eight times, and finally it will be observed that 

 only a very small residue of the bacteria remains and is deposited on 

 the bottom. If such a deposit found in a culture a few weeks old be 

 examined microscopically there will be found a few whole bacilli 

 which still stain well with fuchsin, but the mass of the deposit will 

 be seen to consist of broken-down bacilli which do not stain readily, 

 drops of fat, and crystals. They explain this phenomenon on the 

 ground that the bacteria have produced an enzyme which finally 

 manifests its bacteriolytic action on the germs. If this experiment 

 be repeated with a culture of the bacillus of swine erysipelas, similar 

 changes will be observed. The first effect of the enzyme is to agglu- 

 tinate the bacilli, while the final result is a bacteriolytic one. It 

 will be seen from this that both agglutination and bacteriolysis are 

 supposed to be due to an enzyme produced by the microorganism. 

 The difference between the change observed in artificial cultures and 

 in immune serum is due to the fact that the latter contains a larger 

 quantity of ready-formed enzyme, while in the former the enzyme is 

 gradually produced. Illustrations of similar phenomena are drawn 

 from the study of the action of enzymes on the cellulose of moulds. 

 For instance certain parasitic moulds pierce the wood of living trees 

 by means of enzymes produced in the innermost growths of the mould, 

 1 Zeitachr. f. HygUne, 31, 1899 ; 36, 1901. 



