154 INFECTION AND RESISTANCE 



specifically various specific sensitizers as well as normal antibodies 

 in the serum of the same animal; and this showed that there is no 

 necessity of assuming a variety of specific antisensitizers, as had 

 been done by the German workers. 



As regards the multiplicity of amboceptor or sensitizer, however, 

 though the proof of this, by means of anti-amboceptors, has had to 

 be abandoned, as we have seen, there is still a great deal of evidence 

 advanced in favor of such an assumption. The chief support for 

 such an opinion is found in the "group reactions" among bacteria, 

 similar to those observed for blood cells by Ehrlich and Morgenroth, 

 and described above (see page 151). For it is frequently observed 

 that the antibodies produced by immunization with one species of 

 bacteria may have a certain though lesser degree of action upon 

 other related forms, these in turn absorbing only a part of the ambo- 

 ceptor out of the serum, while the species originally used for im- 

 munization takes out all the amboceptor present. Considering the 

 great chemical complexity of the bacterial or tissue cells, moreover, 

 we may well expect such multiplicity. And it is, indeed, entirely 

 reasonable to suppose that a structure as complex as the bacterial 

 cell may contain a number of antigens and consequently give rise 

 to a number of sensitizers which differ in that each is specific for its 

 particular antigen only. This is merely a restatement of the phe- 

 nomenon of specificity and has, as a matter of fact, no modifying 

 influence on the general principles involved. 



From the point of view of a general understanding of the proc- 

 esses of immunity, however, the question of multiplicity of sensitizer 

 is not so fundamentally important as is the similar controversy which 

 has been waged regarding the unity or multiplicity of alexin or com- 

 plement. Here again there has been some misconception as to the 

 meaning of those who maintain the unity of alexin. Neither Bordet, 

 nor anyone else familiar with experimental conditions, has ever main- 

 tained that the alexins of different animals were functionally iden- 

 tical. It is a well-known fact that the fresh blood sera of various 

 animal species differ from each other considerably in their power to 

 activate bactericidal or hemolytic systems. In regard to hemolysis, 

 fresh guinea-pig serum is very powerful in activating many sensi- 

 tized blood-cell complexes, but weak in activating sensitized guinea- 

 pig corpuscles. Often one finds that the alexin of an animal is en- 

 tirely impotent or but weakly capable of producing hemolysis of the 

 sensitized cells of its own species, though this is not a general rule. 



Again, even without such species relationship, a given alexin may 

 be very weak for certain complexes and strong for others. The 

 alexin of horse blood can even be fixed to sensitized cells 42 without 



42 For the sake of clearness it may be repeated here that by sensitized 

 cells we mean cells which have absorbed specific "amboceptor" or "sensitizer," 

 and have thereby become amenable to the action of alexin or complement. 



