ACQUIRED IMMUNITY. 461 



are very high, these can be still further exalted by artifi- 

 cial means, that is, the natural immunity may be artificially 

 intensified. 



Acquired Immunity in the Human Subject. The following 

 facts are supplied by a study of the natural diseases which affect 

 the human subject. First, in the case of certain diseases, one 

 attack protects against another for many years, sometimes 

 practically for a lifetime, e.g. smallpox, typhoid, scarlet fever, 

 etc. Secondly, in the case of other diseases, e.g. erysipelas, 

 diphtheria, influenza, and pneumonia, a patient may suffer from 

 several attacks. In the case of the diseases of the second group, 

 however, experimental research has shown that in many of them 

 a certain degree of immunity does follow ; and, though we can- 

 not definitely state it as a universal law, it must be considered 

 highly probable that the passing through an attack of an acute 

 disease produced by an organism, confers immunity for a longer 

 or shorter period. The immunity is not, however, to be regarded 

 as the result of the disease per se, but of the bacterial products 

 introduced into the system ; as will be shown below, by suitable 

 gradation of the doses of such products, or by the use of weakened 

 toxins, a high degree of immunity may be attained without the 

 occurrence of any symptoms whatever. 



The facts known regarding vaccination and smallpox ex- 

 emplify another principle. We may take it as practically proved 

 that vaccinia is variola or smallpox in the cow, and that when 

 vaccination is performed, the patient is inoculated with a modified 

 variola (vide Smallpox, in Appendix). Vaccination produces 

 certain pathogenic effects which are of trifling degree as com- 

 pared with those of smallpox, and we find that the degree of 

 protection is less complete and lasts a shorter time than that 

 produced by the natural disease. Again, inoculation with lymph 

 from a smallpox pustule produces a form of smallpox less severe 

 than the natural disease but a much more severe condition than 

 that produced by vaccination, and it is found that the degree of 

 protection or immunity resulting occupies an intermediate posi- 

 tion. The corresponding 'general conclusion from experiments 

 is that the more virulent the organism injected, provided that 

 the animal recovers satisfactorily, the higher is the degree of 

 immunity acquired by it against that organism. Thus in develop- 

 ing immunity of the highest degree the most virulent organisms 



