496 IMMUNITY. . 



more concerned with the cells of the body as destroyers or 

 digesters of bacteria. As regards the subject of antibacterial 

 sera, the results of these two workers may be said to be in 

 harmony in some of the fundamental conceptions. And it is 

 of interest to note that Metchnikoff, starting with the phe- 

 nomena of intracellular digestion, has arrived at the giving off 

 of specific ferments by phagocytes ; whilst Ehrlich, from his 

 first investigations on the constitution of toxins, has arrived at 

 an explanation of antitoxins and immune-bodies also with a 

 theory of cell-nutrition as its basis. Within the last few years 

 marked progress has thus been made towards the establishment 

 of the fundamental laws of 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 pathogenic effects in any animal ; when these are 

 introduced into the body, they rapidly die out. This fact accord- 

 ingly 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 insusceptibility to, or power of neutral- 

 ising, 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 multi- 

 plying in the living tissues without producing local or general 

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

 from this that if the toxins are completely neutralised or ren- 



