ARTIFICIAL IMMUNITY 513 



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 

 ln-li\v, 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 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 natural disease but a much more severe con- 

 dition than that produced by vaccination, and it is found that 

 the decree of protection .or immunity resulting occupies an 

 intermediate position. 



I iniinniity and Recovery from Disease. Recovery from an 

 acute infective disease shows that in natural conditions the virus 

 may be exhausted after a time, the period of time varying in 

 different diseases. How this is accomplished we do not yet 

 fully know, but it has been found in the case of diphtheria, 

 typhoid, cholera, pneumonia, etc., that in the course of the 

 disease certain substances (called by German writers Antikorper) 

 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 immunisation would appear to be going on during the pro- 

 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 ; though the results already obtained make 

 this probable. 



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 

 'I'-fitte 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 



33 



