162 BACTERIA IN RELATION TO DISEASE 



tissues, were found after a short time to be plugged full of leucocytes. The 

 immigration of these was attributed to chemotaxis, sometimes, it is true, 

 without due consideration of the necessities of Weber's law (p. 80). Since 

 bacterial foci in the tissues are almost always surrounded by a crowd of 

 leucocytes, and since these are laden with bacteria, Metschnikoff's theory 

 seemed well founded. 



There was, however, a discovery made respecting the serum of the 

 blood that seemed to show that phagocytosis was neither the only, nor 

 the most important, means of defence that the body possesses against 

 invading bacteria. If bacteria were mixed with blood-serum and this 

 serum examined from time to time by the plate culture method, it was 

 found that fewer and fewer colonies of bacteria arose, until, after an interval 

 of some hours, the serum was sterile, the bacteria being all dead. This 

 bactericidal influence of blood-serum, which is apparently exerted upon 

 all bacteria, is destroyed by the addition of distilled water or by heating 

 the serum for an hour to 55 C. According to Buchner it is due to certain 

 substances, alexines (146) (defensive substances), which, like the toxines, are 

 very unstable, and have not yet been obtained in a pure state. Seeing that 

 serum which has been rendered inactive by distilled water can be restored 

 to bactericidal power by the addition of 0-3 per cent. NaCl or other salts, 

 there is evidently many a mystery hidden in the ' alexines.' 



In the early eighties, when the alexine theory and the theory of 

 phagocytosis were in sharp opposition, most weight was laid upon the 

 destruction of the bacteria. Afterwards, however, the toxine theory of 

 infection (142) was brought forward, and the question arose as to the 

 neutralization of bacterial poisons and the presence of ' antitoxines.' Since 

 bacteria of the most various kinds could be destroyed by one and the 

 same poison, it was not necessary to assume the existence of specific 

 alexines. The different toxines, on the other hand, seemed to require 

 different antitoxines, just as almost each different poison requires a specific 

 antidote. 



Since the animal body is the most delicate, and at present, indeed, 

 the only reagent we possess for bacterial toxines (which, moreover, we do 

 not know in a pure state), it is evident that the antitoxic properties of 

 the blood-serum can only be investigated by means of experiments on 

 animals. 



As in the case of the bacterial poisons, it has not yet been possible 

 to isolate the antitoxines. They are said to be somewhat more resistant 

 than the toxines. But the statements of different investigators differ even 

 as regards the same antitoxines, since they can be examined of course only 

 in a dissolved state in the serum. 



Since the cell-free serum of the blood is a lifeless fluid, we are forced 

 to regard the leucocytes as the producers and carriers of those substances 



