492 IMMUNITY. 



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 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 pos- 

 sessed by an animal of destroying the organisms when 

 introduced into its tissues. It might also, however, be 

 due to an insusceptibility 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 patho- 

 genic bacteria. No doubt, as we have seen, the power of 

 toxine production 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 or- 

 ganisms had especially a power of destroying the normal 

 preventive power resident in the blood and tissues. There 

 is, however, no such thing known as an organism multiply- 

 ing in the living tissues without producing local or general 

 changes, though, theoretically, there might be. We may 



