424 IMMUNITY. 



varying degrees, and accordingly the use of the term has 

 a correspondingly relative significance. Such a thing as 

 absolute immunity is scarcely known, just as we have 

 seen is the case with absolute susceptibility. This is not 

 only true of infection by bacteria, but in the case 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. This statement is well 

 illustrated in the case of the great resistance to the toxines 

 of tetanus possessed by the common fowl. This animal 

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



