CLASS IMMUNITY 129 



is the marked affinity which exists between the meningococcus and 

 the meninges, of the pneumococcus for pulmonary tissue, of strepto- 

 cocci for serous membranes, of the typhoid bacillus for lymphoid 

 structures, etc.; while the other tissues show a more or less well- 

 defined immunity. Evidently the degree of resistance or immunity 

 which the animal offers to infection depends both upon the nature 

 of the organism and the route by which it is introduced. 



If infection, by what we may term a natural route, is excluded, 

 then there will be an apparent immunity, at least, to the organism 

 in question. For practical purposes this type of immunity may, 

 indeed, be regarded as absolute. But that it is not so of necessity 

 can in some instances be demonstrated by introducing the organism 

 through channels by which natural infection would not be likely 

 to occur. In the human being, typhoid infection will thus almost 

 always develop by way of the intestinal canal. In most of our labor- 

 atory animals, infection by this channel is impossible, and we might 

 accordingly regard them as immune. That this is only apparently 

 the case, however, can be readily shown by injecting the organisms 

 intraperitoneally, when a fatal infection can be produced at will. 

 The fact remains, nevertheless, that the various animals in their 

 natural state do not contract typhoid fever, although they must 

 be exposed to infection on many occasions. They may hence be 

 regarded as practically immune. Many instances of immunity no 

 doubt are dependent upon such causes, viz., upon the existence 

 of immunity of those tissues by which natural infection would 

 ordinarily occur. 



Class Immunity. Generally speaking, the natural susceptibility 

 to infection by microorganisms differs with the different classes 

 of animals, with different genera, with different species, and even 

 with different varieties and individuals. We accordingly recognize 

 a natural class immunity, a natural generic and species immunity, 

 natural race immunity, and individual immunity. Class immunity 

 is especially interesting because it presents examples of absolute 

 immunity, under natural conditions at least, which is, after all, 

 exceedingly rare. The immunity of cold-blooded animals toward 

 the majority of those organisms which are pathogenic for warm- 

 blooded animals belongs to this order. But even here the immunity 

 is sometimes only relative and apparent. The frog is thus naturally 

 insusceptible to anthrax, and the injection of large numbers of such 

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