IMMUNITY AND ANAPHYLAXIS 199 



case, the leucocyte itself will disintegrate. The substance 

 present in normal blood, which is bactericidal, Metchnikoff 

 calls " Cytase," and he holds that it is secreted by the 

 phagocytes. The substance formed in immunization which, 

 like the cytase, is bactericidal, he calls the " Fixateur," 

 and also looks upon as a derivative of the leucocytes. He 

 believes that these substances (at least, the cytase) are only 

 set free in the blood-stream by the destruction of the 

 phagocytes. The action of opsonins and leucocyte extract 

 all tend to confirm the importance of phagocytosis, and 

 the probability that these cells, retaining the characters of 

 the amoeba, retain also its marvellous adaptability, which 

 is not usually seen or expected of the fixed tissue cells, 

 which are so very highly specialized as to function. 

 Metchnikoff therefore believes that for every infection 

 the leucocytes develop a power of resistance, which may be 

 revived on any subsequent infection, and so protect to 

 a greater or less degree. 



Ehrlich's Theory is more complex. The discovery 

 of antitoxins led to explanations of their action. At first 

 they were thought to destroy the toxin, but this simple 

 explanation was set aside by the experiments of Calmette 

 on snake poison, which is thermostabile up to ioo C. He 

 noted that non-toxic mixtures of the toxin and antitoxin 

 became toxic again on heating, the inference being that 

 the toxin was bound or inactivated by the antitoxin, 

 which is destroyed on heating above 6o C, and so the 

 more stabile toxin is again left free. Further, C. J. Martin 

 and Cherry demonstrated the close resemblance of the 

 union to that of definite organic compounds, by an 

 experiment in which they tried to pass toxin-antitoxin 

 mixtures through a Chamberland bougie, the pores of 

 which were filled with gelatin. In previous experiments 

 they found that under 50 atmospheres of pressure, toxin 

 passed through but antitoxin did not. In toxin-antitoxin 

 mixtures, if filtered at once, all the toxin came through ; 

 but after standing for variable periods, less came through 

 the longer the time, until two hours after mixing, no toxin 

 passed through the filter (or dialyser) . Then Ehrlich 

 showed, using ricin and antiricin, that definite quantitative 

 proportions of the toxin and antitoxin entered into the 



