IMMUNITY. 223 



groups; in other words, a haptophore for cells and a hapto- 

 phore capable of uniting with a body containing a ferment- 

 producing group. Receptors for lysins are therefore called 

 amboceptors, or receptors with two haptophores (Fig. 52). 

 They are also called receptors of the third order. The sub- 

 stance which reactivates the lysin, the fresh serum, is called 

 complement, and it must be composed of a haptophore in 

 order to attach itself to the amboceptor, and a zymotoxic 

 group in order to produce lysis. On heating fresh normal 

 serum to 55 or 56 C. the complement which it contains is not 

 destroyed, but its zymotoxic group alone is destroyed; the 

 haptophore group, on the other hand, resists heat. So if 

 heated complement be added to inactivated lysin, it unites 

 with the freed haptophore. A lysin inactivated by heat with 

 fresh serum added disintegrates homologous cells; but a lysin 

 inactivated by heat when heated fresh serum is added will not 

 only not produce lysis of homologous cells, but will not do so 

 even when unheated fresh serum is subsequently added. 



The behavior of mixtures of toxins and antitoxins is most peculiar, for they 

 do not in all cases obey the simple rule of relative proportion. It is true that 

 if a certain amount of antitoxin neutralizes a certain amount of toxin, then any 

 multiple of this amount of antitoxin will neutralize the same multiple of toxin if 

 the two are mixed all at once. So far the rule is simple. But if 100 doses of 

 toxin i. e., enough to kill 100 guinea-pigsis exactly neutralized, and then the 

 amount of free toxin necessary to kill a guinea-pig is added, it will not kill a 

 guinea-pig as would be expected. Many doses have to be added, sometimes 

 as much as thirty or forty doses or more, before the mixture again becomes 

 lethal. 



Another remarkable property is that toxin that has stood for a long time 

 loses greatly in poisonous properties, but not in its power of combining with 

 antitoxin. Furthermore, this old toxin will produce antitoxin if injected into 

 horses or other susceptible animals. 



In order to explain these extraordinary reactions several theories have been 

 advanced, and in this connection certain peculiar reactions obtained with 

 lysins, agglutinins and precipitins, which have helped to give an insight into 

 the processes involved, have also been explained on similar theoretical grounds. 

 These theories will now be discussed. 



Ehrlich regards the beef-broth from a diphtheria or tetanus culture as a 



