NATUKAL IMMUNITY 591 



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 im- 

 portance which he attached to the leucocyte. It, however, does 

 not afford explanation of the multiplicity and specificity of 

 antitoxins ; 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 Metchnikoff, 

 starting with the phenomena of intracellular digestion, arrived 

 at the giving off of specific ferments by phagocytes ; whilst 

 Ehrlich, from his first investigations on the constitution of 

 toxins, reached an explanation of antitoxins 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. 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 



