308 STUDIES IN IMMUNITY. 



Antibodies belong simply to a group of active substances in sera. 

 The only affair of the treated animal is to react against certain or 

 all members of this group, representing, as they do, an alien sub- 

 stance. Nor must it be lost sight of that in injecting immune 

 serum we inject, not only specific antibody, but normal serum as well. 

 It is simply the unaccustomed presence of all these foreign sub- 

 stances that leads the animal to form an antagonistic substance, 

 and as a result this substance is not strictly specific. Such an 

 antiserum may be used to protect bacteria or various kinds of 

 blood cells against the respective antibodies obtained from the 

 same animal species. Since the antibodies obtained by immunizing 

 with normal serum will neutralize any antibody from the animal 

 species corresponding to the normal serum employed, it is 

 obvious that it acts on the entire category of substances in this 

 serum. 



Theoretically, at least, we might expect that an animal would 

 react more violently on injection of toxic immune serum specific 

 for its own blood cells.* This condition, however, is not indis- 

 pensable in order to produce a reaction, as is reasonable when 

 we consider that normal serum frequently has a harmful effect on 

 animals of a different species. 



When, however, an animal is injected with a specific immune 

 serum acting on bacteria or cells that have nothing in common with 

 the vaccinated animal, and as a result an antiserum is formed that 

 neutralizes the injected antibody, there is no legitimate reason 

 for us to conclude that there is any necessary identity or even relation 

 in the composition of this animal's cells and the substances (bac- 

 teria, cells, or bacterial products) against which the serum in- 

 jected is specific. There is no reason for introducing the con- 

 ception of common receptors. As a matter of fact the antiserum 

 obtained under these conditions does not differ from that 

 obtained on injecting normal serum; we repeat that it acts indis- 

 criminatingly on any of the active substances present in the foreign 

 serum. 



We believe, then, that the rapid disappearance of passive immunity 



* It may be noted, however, that Kraus and Eisenberg did not obtain any 

 detectable active antiserum on injecting dogs with an immune serum active for 

 dog corpuscles. 



