28 THE PRINCIPLES OF IMMUNOLOGY 



sary, because even although the agglutinating and precipitating 

 properties are destroyed by heat or other means, nevertheless, there 

 remains a group capable of entering into combination with the anti- 

 genie substances, so that the addition of a complete agglutinin or 

 precipitin produces no effect. The third category of Ehrlich includes 

 the receptor or haptine which has been named by Ehrlich the ambo- 

 ceptor, and by Bordet the sensitizer. In this instance the receptor 

 produced by the cell is conceived as a body possessing two com- 

 bining groups, one serving to combine with the antigen and the 

 other serving to combine with complement. These two groups have 

 been called the cytophilic group and the complementophilic group. 

 The complement is a thermolabile substance which has little or no 

 capacity for combining with the antigen. Accepting Ehrlich's hy- 

 pothesis, this haptine of the third order constitutes an intermediary 

 body through the action of which the complement is brought into 

 contact with the cells, be they bacterial or animal, so as to lead to 

 solution. Bordet, however, believes that the immune body enters 

 directly into combination with the antigen, thereby " sensitizing " 

 it so that the complex is operated upon by the complement, or as he 

 calls it, the alexin. The discovery of the phenomenon of comple- 

 ment-fixation demonstrated that a similar substance may operate in 

 the presence of dissolved protein and complement, so as to engage 

 the complement in such a fashion that it is not available for other 

 reactions. In these reactions the participation of the complement is 

 an essential and necessary condition of the reaction. The original 

 Ehrlich theory could not consider the subsequently discovered 

 opsonin or tropin. This substance prepares bacteria and other 

 cells for phagocytosis. It was at first supposed to be a simple 

 immune substance, but as the study of its activity progressed it was 

 found that the presence of complement increases its activity, al- 

 though this latter body is not essential and necessary. We, there- 

 fore, propose to consider the opsonin as belonging essentially to the 

 haptines of the third order. The way in which this differs from the 

 original haptine of the third order is simply in the fact that com- 

 plement may or may not be utilized in the reaction. In order to 

 differentiate we suggest that the amboceptors of Ehrlich be looked 

 upon as " obligate " amboceptors and the opsonin be regarded as a 

 " facultative " amboceptor. 



Recent Criticism of the Ehrlich Hypothesis. Following the 

 earlier discoveries of immune phenomena numerous studies were 

 made of the chemistry of immune substances and immune reactions. 

 These will be discussed in the chapters on the special immune reac- 

 tions. It may be said at this time that many objections have been 

 raised to the Ehrlich hypothesis, particularly as the study of physi- 

 cal chemistry, more particularly that part which refers to colloids, 

 has advanced. As will be seen from the brief review of the Ehrlich 

 hypothesis given above, this investigator was much influenced by 

 the status of chemistry which prevailed when he announced his 



