ACQUIRED IMMUNITY IN THE HUMAN SUBJECT. 459 



have seen is the case with absolute susceptibility. This is 

 not only true of infection by bacteria, but of toxines also ; 

 when the resistance of an animal to these is of high 

 degree, the resistance may be overcome by a very large 

 dose of the toxic agent. For example, the common fowl 

 may be able to resist as much as 20 c.c. of powerful tetanus 

 toxine, but on this amount being exceeded may be affected 

 by tetanic spasms (Klemperer). On the other hand, in 

 cases where the natural powers of resistance are very high, 

 these can be still further exalted by artificial 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 

 cannot definitely state it as a universal law, it must be 

 considered highly probable that the attack of an acute 

 disease produced by an organism, confers immunity for a 

 longer or shorter period. 



The facts known regarding vaccination and smallpox 

 exemplify 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 inocu- 

 lated with a modified variola (vide Smallpox, in Appendix). 

 Vaccination produces certain pathogenic effects which are 

 of trifling degree as compared 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 



