INFECTION AND IMMUNITY 373 



implied that the cells of the host, especially the white 

 corpuscles of the blood, took the microparasites into their 

 substance and destroyed them just as an amceba takes 

 up many small objects, digests, and dissolves them. 



An investigation of the leucocytes of immune animals 

 shows that though there are notable exceptions, the 

 blood corpuscles do behave in this manner. It is also 

 found, though again there are exceptions, that the cor- 

 puscles of susceptible animals usually neglect the micro- 

 parasites, which are therefore free to multiply; but that 

 when such naturally susceptible animals are by any 

 means made immune, their corpuscles change and 

 begin to take up the microparasites. 



The conditions thus corresponded fairly well with 

 the requirements of the theory until certain additional 

 facts were discovered. 



Thus it was found by Nuttall and Buchner that bacteria 

 are commonly killed when placed in the blood serum 

 of an animal. This led to a new idea that the bacteria 

 were killed by some substance in the body juices and 

 only taken up by the phagocytes after death had made 

 them harmless. Many interesting and some paradoxical 

 observations were made. Thus the bacillus of anthrax 

 when put into the rabbit's body rapidly multiplies, 

 distributes itself throughout the blood, reaches all the 

 organs, and kills the rabbit. If, however, the anthrax 

 bacilli are placed in some of the rabbit's blood drawn 

 into a test-tube, they meet with speedy destruction. 

 Why should this paradox occur? Why should their 

 introduction into the rabbit's blood in the rabbif's body 

 be followed by the death of the rabbit, but their intro- 

 duction into the rabbit's blood in a test-tube be followed 

 by their own death? This was a much debated point. 

 Buchner and his followers named the bacteria-destroy- 

 ing substance of the blood alexine. For a while it seemed 

 as though the doctrine of phagocytosis had received 

 its death-blow, but Metchnikoff replied that the destruc- 

 tion of the bacteria by the phagocytes depended upon 



