THE BLOOD 125 



destroyed the poisons or toxins they produce are in certain other 

 cases neutralised by antitoxins. 



Metschnikoff 's view, which is very widely accepted by bacterio- 

 logists, is that the most stress should be laid upon phagocytosis as 

 the principal factor in tha resistance of the body to bacteria ; and the 

 recent discovery of opsonius by Sir A. E. Wright not only emphasises 

 this opinion, but shows how the body fluids co-operate with the 

 phagocytes in the process. The word ' opsonin ' is derived from a 

 Greek word which means 'to prepare the feast.' Washed bacteria 

 from a culture are distasteful to leucocytes, and would therefore, 

 other things being equal, be pathogenic if injected into an animal's 

 body. But if the bacteria have been previously soaked in serum, 

 especially if that serum has been obtained from the blood of an 

 animal previously immunised against that special bacterium, then 

 the leucocytes devour them eagerly. It was at first supposed that 

 something had been added to the bacterium to make it tasty, and 

 that each kind of bacterium requires its own special sauce or 

 opsonin. It is, however, equally possible that the serum has not 

 added anything to the bacterium, but removed from it something 

 that previously made it distasteful. At any rate the ultimate effect 

 is the same, and the bacterium is rendered non- pathogenic. When a 

 person is attacked with some invading organism, say the tubercle 

 bacillus, if that person's blood is naturally rich in the proper kind of 

 opsonin he will not be troubled with tuberculosis ; but if the opsonic 

 power of his blood is low the organism will produce the disease. 

 The modern treatment of tuberculosis aims at increasing the opsonic 

 power of the blood by improving the general condition of the patient 

 by good food arid pure air, and also by the injection of the appro- 

 priate opsonin into his -blood. 



Lastly, we come to a question which more directly appeals to the 

 physiologist than the preceding, because experiments in relation to 

 immunity have furnished us with what has hitherto been lacking, 

 a means of distinguishing human blood from the blood of other 

 animals. 



The discovery was made by Tchistovitch (1899), and his original 

 experiment was as follows : Babbits, dogs, goats, and guinea-pigs 

 were inoculated with eel-serum, which is toxic ; he thereby obtained 

 from these animals an antitoxic serum. But the serum was not only 

 antitoxic ; it also produced a precipitate when added to eel-serum, 

 though not when added to the serum of any other animal. In other 

 words, not only has a specific antitoxin been produced, but also a 

 specific prccipitin. Numerous observers have since found that this is 



