462 IMMUNITY. 



are ultimately employed. A corresponding principle, with cer- 

 tain restrictions {vide p. 471), obtains in the case of toxins. 



Immtmity 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 prog- 

 ress 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 ; and it is by no means the case that 

 the degree of immunity (active) is always proportional to the 

 amount of these substances in the blood. 



Artificial Immunity. 



Varieties. — A number of facts regarding immunity have 

 been given in the description of the pathogenic organisms in 

 previous chapters. We shall here give a general systematic 

 description of the methods, and discuss the principles involved. 

 According to the means by which it is produced, immunity may 

 be said to be of two kinds, to which the terms active zxiA. 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 

 at suitable intervals the dose of organisms or of the products 

 can be gradually increased ; or, what practically amounts to the 

 same, an organism of greater virulence or a toxin of greater 

 strength may be used. A proportionate degree of resistance or 

 immunity can thus be developed, which degree in course of time 



