PROPERTIES OF ANTISENSITIZERS. 307 



anti-antibodies are not always detectable experimentally, which 

 may account in part for the variation in results. There are different 

 affinities to be considered in these experiments. There is the 

 affinity of the anti-antibody for the specific antibody and for 

 the normal antibody, and then there is the affinity of the antibody 

 for the sensitive cell used as reagent. The susceptibility of this 

 reacting cell must also be taken into account. The net result from 

 these factors varies, and yet is the only result that we take into 

 consideration. The laws that govern these phenomena may in 

 reality be general, although the apparent results differ. The study 

 of immunity is full of such instances. It is a general rule that the 

 injection of bacteria gives rise to sensitizers, and these sensitizers 

 increase the destructive power of the alexin. It is exceptional, 

 nevertheless, that the immune serum obtained in this way is strongly 

 bactericidal for the specific bacterium; and why? It is due to the 

 fact that, although there are certain bacteria that are so delicate 

 as to be destroyed by immune sera, the majority of them are resist- 

 ant and suffer no injury from the serum in a suitable medium, as 

 Metchnikoff has shown by numerous examples. 



When we inject animals with immune serum in the hope of 

 obtaining an anti-antibody we naturally think of the animal as 

 reacting particularly to the antibody that we are studying. If 

 the experiment succeeds, as is the case with cholera serum, and if 

 we believe in Ehrlich's theory, we conclude that the animal 

 has formed an anticholera serum owing to the reproduction of 

 receptors analogous to those of the cholera vibrio. But this con- 

 ception does not represent the true state of affairs. If the cholera 

 sensitizer is neutralized, it is not on account of its specific 

 peculiar qualities and simply because it affects the cholera vibrio. 

 If it is neutralized it is not owing to any " personal equation," so 

 to speak, but simply because it belongs in common with all sensi- 

 tizers (and probably also all antitoxins) to a group of substances 

 belonging to species A, which, on injection into species B, cause a 

 reaction. To speak colloquially, we might say that the injected 

 animal is not concerned as to whether it is given antibodies that 

 affect tetanus or diphtheria toxin, or those acting on cholera or 

 typhoid bacilli ; haptophore groups of toxins and receptors of bacilli 

 do not particularly appeal to it. 



