Immunity against pathogenic micro-organisms 159 



point menaced. These cells readily exert their phagocytic function 

 and rid the organism of the introduced bacteria. In order to obtain 

 a complete grasp of the part played by this reaction it will be found 

 useful to inject beneath the skin of one ear of a rabbit a little anthrax 

 vaccine and beneath the skin of the other the same quantity of virulent 

 bacilli. The difference between the reaction in the two cases is very 

 striking. The ear inoculated with vaccine soon becomes the seat of a 

 circumscribed inflammation with a purulent exudation, all the bacilli 

 in which have been devoured by the leucocytes. The other ear, on 

 the contrary, presents, around the injected virus, only a serous or 

 blood- tinged exudation containing no, or few, leucocytes ; the bacilli 

 are found free in the liquid and multiply without let or hindrance. 

 Meeting with no opposition the virus becomes generalised through- 

 out the organism and brings on death by anthrax septicaemia. 

 Rabbits, into which anthrax vaccines only are introduced, oppose to 

 the invasion of the bacilli a leucocytic barrier which arrests their 

 extension. The natural immunity of the sheep, rabbit and guinea- 

 pig is also a phagocytic immunity, but it is only capable of being 

 exercised against bacilli previously attenuated in virulence. The [169] 

 researches of Mme Metchnikoff^ on the reaction of the phagocytes 

 of these animals to the bacilli of Pasteur's two anthrax vaccines have 

 demonstrated the importance of the destruction of these bacilli by 

 the leucocytes. All the other examples of natural immunity against 

 anthrax are also merely relative. The fowl that resists an anthrax 

 virus strong enough to kill an ox or a horse, succumbs to a special 

 variety of anthrax cultivated by Levin 2. The dog, as we have seen, 

 in spite of its pronounced natural immunity against anthrax, is kiUed 

 by the special anthrax bacillus prepared by Martel. 



In this immunity against anthrax we have to deal with a bacillus 

 capable of living and reproducing itself in extremely varied media. 

 Hence the reason, it may be said, that the bactericidal influence 

 of the fluids is so little pronounced in this case. To bring it into 

 relief we must, therefore, choose a bacterium less capable of adapting 

 itself to the chemical composition of various culture media. In this 

 matter we cannot do better than select pathogenic spirilla of ex- 

 tremely delicate nature and analyse the mechanism of the natural 

 immunity of certain species of animals with respect to them. It 

 must not be forgotten, however, that here we are making use of 



1 Ann. de VInst. Pasteur, Paris, 1891, t. v, p. 145. 

 ^ ' Om Mjaltbrand hos Hons," Stockholm, 1897. 



