GENERAL PHENOMENA OF IMMUNITY 27 



strates somewhat variable activities. For example, it was found 

 that antitoxins operate differently from other immune substances; 

 that agglutinins and precipitins operate in a special fashion which 

 is practically identical for both substances; and that cytolysins, includ- 

 ing the lytic bodies for bacteria as well as for animal cells, require 

 the presence of fresh serum containing the so-called complement or 

 alexin. The specific cytolysins were found to be similar to certain 

 other substances which are now referred to as complement-fixing 

 bodies. Finally the discovery of opsonins and tropins showed that 

 there is in all probability a fourth group or sub-group of these 

 immune substances. 



The Ehrlich Classification. Ehrlich, on the basis of the general 

 outline given above, divided the immune bodies into three groups, 

 depending upon demonstrable differences in their nature. He found 

 that the receptors in some instances are not immunologically simple 

 bodies, but that even in this sense they show varying degrees of 

 complexity. In the more complex forms the actual receptor or 

 combining groups constitute only a part of the immune substances, 

 and he therefore applied a more comprehensive term, the haptines. 

 He included in the haptines of the first order the antitoxins, in the 

 haptines of the second order the agglutinins and precipitins, and in 

 the haptines of the third order the cytolysins and other amboceptors. 

 The early studies of antitoxins made it appear that the neutralizing 

 effect of these substances was similar to the neutralizing action of 

 alkalies and acids, but it was subsequently discovered that such 

 combinations may be, at least in part, dissociated. It was then 

 found that toxin may undergo a variety of changes as the result of 

 preservation. Subsequently it was learned that the antitoxin would 

 combine not only with the toxin, but with its degeneration products. 

 This has complicated the conception considerably, and we may say 

 in brief that according to the Ehrlich conception the antitoxin con- 

 stitutes a simple receptor or combining group capable of entering 

 into combination with a special combining group in the toxin called 

 the haptophore. In order to account for the combination of anti- 

 toxin with the degeneration products of toxins it was necessary to 

 assume that the toxin exhibits its essential combining property in 

 the haptophore group and that the toxin also possesses a toxophore 

 group, serving to give it its poisonous character and partly de- 

 stroyed during preservation or by certain degrees of heat. The 

 second category of Ehrlich includes the agglutinins and precipitins. 

 In the study of these substances it was found that the agglutinins 

 and precipitins may be deprived of the agglutinating and precipitat- 

 ing properties by preservation or by the application of certain de- 

 grees of heat. Ehrlich, therefore, conceived the idea that in this 

 instance we have to deal with a somewhat more complicated haptine 

 containing a combining group and a so-called zymophore group, 

 the latter leading to the special reaction. These constitute the re- 

 ceptors or haptines of the second order. This assumption is neces- 



