IMMUNITY, IMMUNIZATION, AND CURE. 65 



The blood-serum of an animal that has been immunized 

 according to one or the other of the methods already men- 

 tioned is capable of conferring immunity upon a susceptible 

 animal not previously treated. The same property that 

 belongs to the blood-serum is possessed by all of the tissue- 

 fluids, as well as the milk and the egg-yolk, of highly im- 

 munized animals. The most significant feature of this 

 method of immunization is the simplicity and the rapidity 

 with which it confers immunity. The immunity is, appar- 

 ently, induced immediately as a result of the serum-injection, 

 and no such phenomenon is observed as recovery from a 

 disease marking the development of immunity. On the 

 other hand, the immunity conferred by the injection of 

 serum lasts only a few weeks, and is thus of much shorter 

 duration than that induced more slowly through bacterial 

 activity. Ehrlich has designated as passive the immuniza- 

 tion that is effected by means of serum and in which the 

 affected organism passes through no disease, in contradis- 

 tinction from all other active methods of immunization in 

 which a more or less severe disease must be passed through. 

 Immunization by means of serum thus is mediate (indirect), 

 in contrast with immediate (direct] immunization by means 

 of bacteria and their products. To what extent we have 

 gained a comprehension of the processes that take place 

 in the act of immunization with serum will be discussed 

 later. Here may be mentioned only the one direction in 

 which this method of immunization has attained some 

 degree of importance : it permits, in certain cases, the appli- 

 cation of a harmless test as to the existence of immunity. 

 Whereas formerly this could only be determined by induc- 

 ing infection, at present a small amount of blood is obtained 

 from the individual under examination, and observation is 

 made to determine if the serum is capable of conferring 

 immunity upon a susceptible animal. In this way it has 

 become possible to decide with experimental certainty as 

 to the existence or not of immunity also in human beings. 

 It must not, however, be forgotten that the blood-serum is 

 by no means always an index of the presence or absence 

 of immunity. Behring refers to horses highly protected 

 against tetanus, whose blood-serum exhibited no protective 

 activity. Upon the other hand, there are animals whose 

 serum possesses marked immunizing properties, and which 

 themselves succumb to the mildest infection. Metschnikoff 



