INFECTION AND IMMUNITY. 571 



the immediate destructive activity of its " zymotoxic " 

 group. 



It is interesting to note in connection with this 

 hypothesis, that both " receptors " and " complements " 

 are present in normal susceptible, as well as in immune 

 animals, but that during immunization only the " recep- 

 tors" are multiplied as a result of the specific stimula- 

 tion necessary to the establishment of immunity. 



According to the nature of the intoxicant from which 

 the animal is immunized, the one or the other of the 

 structurally and functionally different tyjjes of receptors 

 is increased — i. e., in immunity from a simple toxin the 

 simplest type of receptor appears in the blood (receptors 

 of the first order, Ehrlich) ; in immunity that is asso- 

 ciated with agglutinating or preciptating powers on the 

 part of the blood-serum receptors having a haptophore 

 and a zymophore group appear (receptors of the second 

 order) ; while in immunity from such molecular com- 

 plexes as blood-, tissue-, or bacterial cells there are 

 produced receptors of the third order, which act through 

 their haptophore groups as intermediate links between the 

 body to be destroyed and the normally present ferment- 

 like complement that is to bring about the destruction. 

 For all the foreign irritants from which animals have 

 been immunized, be it alien blood, tissue-cells, milk, or 

 bacteria, there is assumed to be circulating normally in 

 the blood a "complement" specifically related to that 

 irritant on the one hand, and to its " receptor " on the 

 other. This idea of plurality for the complement is 

 apparently the vulnerable point in the argument. At 

 all events, it has been vigorously assailed by Bordet and 

 Buchner, especially, who consider the complement as a 

 unit, and who do not regard it as possessed necessarily 



