AKTIFICIAL IMMUNITY 553 



of an acute disease 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 intro- 

 duced 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. It has been 

 found in the case of diphtheria, typhoid, cholera, pneumonia, etc., 

 that in the course of the disease certain substances appear in the 

 blood, which are antagonistic either to the toxin or to the vital 

 activity of the organism. In such cases a process of immunisa- 

 tion would appear to be going on during the progress of the 

 disease, and when this immunisation has reached a certain height, 

 the disease naturally comes to an end. It cannot, however, be 

 said as yet that such antagonistic substances are developed in 

 all cases ; although the results already obtained make this 

 probable. 



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 inoculated 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 naturar 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 

 position. 



Artificial Immunity. 



Varieties. — According to the means by which it is produced, 

 immunity may be said to be of two kinds, to which the terms 

 active and passive are generally applied, or we may speak of 

 immunity directly, or indirectly, produced. We shall first give 

 an account of the established facts, and afterwards discuss some 

 of the theories which have been brought forward in explanation 

 of these facts. 



Active immunity is obtained by (a) injections of the organisms 

 either in an attenuated condition or in sub-lethal doses, or (b) 

 by sub-lethal doses of their products, i.e., of their " toxins," the 

 word being used in the widest sense. By repeated injections 



