498 IMMUNITY 



that leucocytes may also be concerned in the production of 

 antitoxins. Apart, however, from antitoxin formation, he con- 

 siders the acquired resistance of the cells themselves of high 

 importance in toxin immunity. 



When we consider MetchnikofFs theory as thus extended to 

 cover recently established facts, it must be admitted that it affords 

 a rational explanation of a considerable part of the subject, 

 though the elucidation of the chemiotactic phenomena during 

 immunisation as explained above detracts from the importance 

 which he attached to the leucocyte. It, however, does not afford 

 explanation of the multiplicity and specificity of antitoxins as 

 Ehrlich's does ; on the other hand, it is 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 funda- 

 mental conceptions. And it is of interest to note that Metchni- 

 koff, starting with the phenomena 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 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 



