IMMUNITY AND SUSCEPTIBILITY . 705 



blood corpuscles) and those substances which have a lytic action on 

 various body cells. The substances which act on the bacteria are 

 called bacteriolysins, those acting on erythrocytes are called hemolysins, 

 and those acting on the other body cells are called cytolysins. The 

 mechanism of these lytic processes is quite complex. It should also 

 be noted in this connection that there are certain substances which 

 kill or seriously injure bacteria and body cells and do not actually 

 disintegrate them. Such chemical bodies are designated respectively 

 as bactericidal substances and cytotoxins. 



The first observations in regard to bactericidal and bacteriolytic 

 substances were made by Nuttall and later by Biichner. Biichner 

 noted these substances in normal sera and other body fluids and 

 named them alexins (Gr. to guard). He assumed that they were 

 concerned in the immunity of the body. This is not necessarily true 

 as certain blood sera are frequently highly bactericidal when the fin- 

 dividual is relatively susceptible. This is true of human blood serum 

 and B. typhosus. Furthermore, in certain instances the animal is 

 immune to the disease and the serum is not in any sense bactericidal. 

 This is the case with the dog and Bact. anthracis. 



Pfeiffer a number of years ago observed that when Msp. comma of 

 Asiatic cholera was introduced into the peritoneal cavity of the normal 

 guinea-pig the bacteria underwent lysis. He also noted that the 

 process was much more rapid in the immune guinea-pig. Pfeiffer had 

 the idea in the beginning that lysis did not take place anywhere but in 

 the body of the animal but later it was demonstrated by a number of 

 men, among them Metchnikoff, that the lytic action would also take 

 place in the test-tube (in vitro). 



Bordet and others later showed that some normal sera possess 

 the power of liberating the hemoglobin in red blood corpuscles. It 

 was also shown that these hemolytic substances could be developed in 

 the body of an animal if that animal were injected or immunized with 

 a suspension of erythrocytes. The phenomenon of hemolysis is easily 

 observed and studied and the amount of the hemolytic agent can be 

 accurately determined as the amount of hemoglobin liberated varies 

 accordingly. The mechanism of hemolysis and bacteriolysis corre- 

 spond exactly and accordingly much about the latter process was first 

 worked out by experimentation with hemolysins. 



Lytic substances can be prepared for a large number of bacteria and 



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