592 IMMUNITY 



and immune-bodies also with a theory of cell-nutrition as its 

 basis. 



NATURAL IMMUNITY. 



We have placed the consideration of this subject after that of 

 acquired immunity, as the latter supplies facts which indicate in 

 what direction an explanation of the former may be looked for. 

 There may be said to be two main facts with regard to natural 

 immunity. The first is, that there is a large number of bacteria 

 the so-called non-pathogenic organisms which are practically 

 incapable, unless perhaps in very large doses,, of producing patho- 

 genic effects in any animal ; when these are introduced into the 

 body they rapidly die out. This fact, accordingly, shows that 

 the animal tissues generally have a remarkable power of destroy- 

 ing living bacteria. The second fact is, that there are other 

 bacteria which are very virulent to some species of animals, 

 whilst they are almost harmless to other species ; the anthrax 

 bacillus may be taken as an example. Now it is manifest that 

 natural immunity against such an organism might be due to a 

 special power possessed by an animal of destroying the organisms 

 when introduced into its tissues. It might also possibly be due 

 to an insusceptibility to, or power of neutralising, the toxins of 

 the organism. For the study of the various diseases shows that 

 the toxins (in the widest sense) are the weapons by which morbid 

 changes are produced, and that toxin-formation is a property 

 common to all pathogenic bacteria. There is, moreover, no 

 such thing known as a bacterium multiplying in the living tissues 

 without producing local or general changes, though, theoretically, 

 there might be. As a matter of fact, however, natural immunity 

 is in most cases one against infection, i.e., consists in a power 

 possessed by the animal body of destroying the living bacteria 

 when introduced into its tissues : such a power may exist though 

 the animal is still susceptible to the separated toxins. We shall 

 now look at these two factors separately. 



1. Variations in Natural Bactericidal Powers. The funda- 

 mental fact here is that a given bacterium may be rapidly 

 destroyed in one animal, whereas in another it may rapidly 

 multiply and produce morbid effects. The special powers of 

 destroying organisms in natural immunity have been ascribed 

 to (a) phagocytosis, and (b) the action of the serum. 



(a) The chief factors with regard to phagocytosis have been 

 given above. The bacteria in a naturally immune animal, for 

 example, the anthrax bacillus in the tissues of the white rat, are 

 undoubtedly taken up in large numbers and destroyed by the 



