Immunity against pathogenic miwo-organisms 129 



A knowledge of all these facts is necessary before we can deal 

 with the subject to which the present chapter must be devoted 

 the innate natural immunity of animals and man against pathogenic [137] 

 micro-organisms. As, under natural conditions, it is the micro- 

 organism and not its toxic products which invades the organism, it 

 is clear that we must give the first place to the study of immunity 

 against the micro-organism. The more so because this form of im- 

 munity is much more frequently met with than is an insusceptibility 

 to toxins. 



Since the animal organism has a very variable composition it 

 might be concluded that the micro-organisms find in the refractory 

 species simply a chemical medium in which they cannot live. We 

 cannot go far in the discussion of this supposition without seeing that 

 it may be rejected. Among the pathogenic micro-organisms some are 

 distinguished by a great fastidiousness and sensitiveness as regards 

 the medium in which they are placed. Such, for example, are the 

 parasites of malaria and their allies. They live inside the red blood 

 corpuscles of Vertebrata and appear to be extremely discriminating 

 in regard to their requirements. All animals, even monkeys, are 

 refractory to human malarial fevers. It might be concluded from 

 this that here at least the immunity may be due to the fact that the 

 chemical composition of the contents of the red corpuscles in the 

 immune animals is different from that of the red corpuscles of man. 

 But when we see, as was first demonstrated by Ross 1 , that the malaria 

 parasite of Laveran, having made its way into the digestive canal 

 of certain mosquitos (Anopheles), there develops abundantly, it is 

 difficult to maintain this thesis. 



Among other micro-organisms of animal origin we have the Try- 

 panosoma, the parasite of the terrible disease propagated by the 

 Tsetse fly which commits such ravages amongst mammals. Man alone 

 escapes it, exhibiting a natural immunity that nothing apparently can 

 overcome. Are we to affirm that it is the difference in the chemical 

 composition of the human body which assures to man his immunity 

 against a parasite that attacks indifferently an herbivorous animal, 

 such as the ox or rabbit, or a carnivorous animal, such as the dog ? 

 In these examples I have chosen merely those micro-organisms which 

 it has never been possible to cultivate on any artificial nutrient 



1 Brit. Mecl Journ., London, 1897, n, p. 1786; 1898, 1, p. 550. Ann. de VInsL 

 Pasteur, Paris, 1899, t. xm, p. 136. 



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