ARTIFICIAL IMMUNITY 457 



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 con- 

 sidered 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 

 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 degree of protection or immunity resulting occupies an inter- 

 mediate position. 



Immunity 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- 

 gress of the disease, and when this immunisation has reached a 

 certain height, the disease naturally comes to an end. It can- 

 not, however, be said as yet that such antagonistic substances 

 are developed in all cases ; though the results already obtained 

 make this probable. 



AKTIFICIAL IMMUNITY 



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

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



