CONDITIONS MODIFYING PATHOGENICITY 177 



if a larger dose be introduced, a fatal peritonitis may follow. 

 Again, a certain quantity of a particular organism injected 

 subcutaneously may produce only a local inflammatory change, 

 but in the case of a larger dose the organisms may gain entrance 

 to the blood stream and produce septicaemia. There is, there- 

 fore, for a particular animal, a minimum lethal dose which can 

 be determined by experiment only ; a dose, moreover, which is 

 modified by various circumstances difficult to control. 



The path of infection may alter the result, serious effects often 

 following a direct entrance into the blood stream. Staphylo- 

 cocci injected subcutaneously in a rabbit may produce only a 

 local abscess, whilst on intravenous injection multiple abscesses 

 in certain organs may result and death may follow. Local 

 inflammatory reaction with subsequent destruction of the 

 organisms may be restricted to the site of infection or may 

 occur also in the related lymphatic glands. The latter 

 therefore act as a second barrier of defence, or as a filtering 

 mechanism which aids in protecting against blood infection. 

 This is well illustrated in the case of "poisoned wounds." In 

 some other cases, however, the organisms are very rapidly 

 destroyed in the blood stream, and Klemperer has found that, 

 in the dog, subcutaneous injection of the pneumococcus produces 

 death more readily than intravenous injection. 



'2. The. Xnhject of Infection. Amongst healthy individuals 

 susceptibility and, in inverse ratio, resistance to a particular 

 microbe may vary according to (a) species, (b) race .and in- 

 dividual peculiarities, (c) age. Different species of the lower 

 animals show the widest variation in this respect, some being 

 extremely susceptible, others highly resistant. Then there are 

 diseases, such as leprosy, gonorrhoea, etc., which appear to be 

 peculiar to the human subject and have not yet been trans- 

 mitted to animals. And further, there are others, such as 

 cholera and typhoid, which do not naturally affect animals, 

 and the typical lesions of which cannot be experimentally 

 reproduced in them, or appear only imperfectly, although 

 pathogenic effects follow inoculation with the organisms. In 

 the case of the human subject, differences in susceptibility to a 

 certain disease are found amongst different races, and also amongst 

 individuals of the same race, as is well seen in the case of tubercle 

 and other diseases. Age also plays an important part, young 

 subjects being more liable to certain diseases, e.g. to diphtheria. 

 Kurt her. at different periods of life certain parts of the body are 

 more susceptible, for example, in early life, the bones and joints 

 to tubercular and acute suppurative affections. 



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