136 THE DIFFERENT TYPES OF IMMUNITY 



Here also our knowledge is as yet very meager, but it would seem 

 that this possibility actually exists. In the case of the anthrax 

 bacillus, for example, it has been ascertained that by suitable methods 

 the organism can be deprived of its power to form capsules and 

 that such strains are then no longer capable of producing infection. 

 We have seen before that this organism owes its infectiousness 

 to a large extent to its ability to surround itself with a capsule, 

 and that when once encapsulated it is no longer open to successful 

 attack by the phagocytes. It is thus easy to see why an animal 

 should prove immune to infection when an organism is introduced 

 which depends for its existence in its new environment upon aggres- 

 sive factors of this order and is incapable of developing them. 



While immunity of this type would depend upon lack of aggres- 

 sivity in the more general sense on the part of an organism, there is 

 evidence to show that immunity may also be due to the same factor 

 in the more restricted sense of Bail, viz., upon an inability of the 

 organism to overcome the normal defensive factors by the secretion 

 or liberation of soluble aggressins. This is well illustrated in the 

 case of the pigeon, which is markedly immune to anthrax even 

 though its serum per se is not bactericidal for this organism. Upon 

 the addition of leukocytes, however, it becomes so, and against 

 this combination an amount of anthrax aggressin is powerless which 

 would suffice to overcome the bactericidal power of a corresponding 

 serum-leukocyte mixture taken from a guinea-pig which itself is 

 markedly antiaggressive in its action. Of the manner in which 

 this effect is produced, however, we know nothing. 



We denote this type of immunity as antiaggressin immunity merely 

 to express the fact that it depends upon factors which are not of 

 a bactericidal nature, but which prevent the development of those 

 aggressive functions upon which certain organisms depend for their 

 existence, after invasion of the body has taken place. The animal 

 is immune not because it has stronger bactericidal forces either in 

 its serum or its cells, not because it can prevent the animalization 

 of the invading organisms, not because of any antitoxic mechanism, 

 but because the organisms for some reason find themselves incapable 

 of exercising their special aggressive forces. But of the reasons why 

 this should be so in one animal and not in another, we know nothing. 



While the existence of an antiaggressin immunity in the special 

 sense of Bail, as just outlined, has thus far been established only 



