452 IMMUNITY. 



phagocytosis has undergone modification. Similar evi- 

 dence with regard to the origin of antitoxic substances is 

 wanting. The whole question, however, is still an open 

 one, and the formation of all these bodies may not be 

 restricted to one class of cells, but may take place more 

 generally. 



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 pathogenic 

 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 

 destroying 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, however, be due to an insus- 

 ceptibility to, or power of neutralising, the toxines of the 

 organism. For the study of the various diseases shows 

 that the toxines (in the widest sense) are the weapons by 

 which morbid changes are produced, and that toxine- 

 formation is a property common to all pathogenic bacteria. 

 No doubt, as we have seen, the power of toxine pro- 

 duction does not go hand in hand with the power of 

 multiplying throughout the body. In the case of organ- 

 isms which multiply in the blood and produce septicaemia, 

 the amount of toxine formed relatively to the number of 

 the organisms is small, and it would appear as if these 



