298 INFECTION AND RESISTANCE 



a small amount of culture material, together with agar, broth, or 

 any foreign substance which may inhibit or divert phagocytosis from 

 the spores, is injected into these animals rapid proliferation and 

 death with toxemia result. If, on the other hand, the spores are 

 carefully washed of foreign material and toxin rapid phagocytosis 

 results and the animals recover. 



The parallelism which was followed out so extensively between 

 natural immunity and phagocytosis was even more closely marked 

 in the case of artificially acquired immunity. The first observations 

 of this kind made by Metchnikoff, again on the subject of anthrax 

 infection, were carried out by the active immunization of rabbits. 

 The subcutaneous injection of virulent anthrax bacilli into normal 

 rabbits is usually followed by a rapid growth of the bacteria, with 

 much serous exudation but hardly any leukocytic accumulation. In 

 immunized animals, on the other hand, the bacilli are taken up by 

 hosts of phagocytes, just as this occurs in naturally resistant dogs or 

 other animals. Similarly Bordet 6 has shown that cholera spirilla 

 injected into the blood stream of cholera-immune animals are taken 

 up by leukocytes even before they can be subjected to lysis by the 

 circulating lytic antibodies. 



It would add little to clearness were we to multiply the examples 

 in which it has been demonstrated that the acquisition of increased 

 resistance is accompanied by enhancement of the phagocytic process. 

 This statement may be regarded as an axiom, and indeed our later 

 discussions of the opsonins and bacteriotropins will show clearly why 

 such a state of affairs is to be expected. Taken by itself, however, 

 it does not necessarily prove that the destruction of the invading 

 germs is entirely due to the leukocytes. It might still be possible 

 that the bacteria are injured or even killed by the antibacterial 

 serum constituents before they can be taken up and carried away 

 by the cellular elements; the phagocytes then would act only as 

 scavengers for the removal of the dead bodies. Indeed, this opinion 

 was long held by a number of the adherents of the purely humoral 

 school. However, such a point of view is no ^nger tenable espe- 

 cially in the light of the later opsonin studies just referred to. 

 Moreover, long before these more recent studies it was clear that 

 bacteria may often grow within the leukocytes finally destroying 

 these and that they may even remain fully virulent after ingestion. 

 For, as Metchnikoff showed, if guinea pigs were injected with a 

 little of the exudate formed after the injection of anthrax bacilli 

 into immunized rabbits (an exudate in which there were no longer 

 any extracellular bacteria because of energetic phagocytosis) death 

 often resulted. It was clear, therefore, not only that the ingested 

 bacteria were still alive, but that they were, at least in part, still 

 fully virulent. 



6 Bordet. Ann. de I'Inst. Past., Vol. 9, 1895. 



