GENERAL PHENOMENA OF IMMUNITY 29 



views. The idea predominated that the immune reactions resemble 

 the more or less fixed changes which are seen in the chemical reac- 

 tions of crystalloids. As it was found that practically all immune 

 substances are colloidal in nature and either are proteins or are very 

 closely related to proteins the similarity of the immune reactions to 

 colloidal reactions became more and more strongly emphasized. In 

 fact, in a general way, practically all immune reactions parallel in 

 their general phases similar demonstrable reactions with colloids. 

 There is but one feature of immune reactions which has not yet 

 been explained on the basis of colloid chemistry, namely, their 

 specificity. This does not mean, however, that further investiga- 

 tion will not clear up this phase of the problem. It is but fair to say 

 that the Ehrlich hypothesis provides an excellent basis for the classi- 

 fication of immune phenomena, but as will be shown subsequently, 

 the conception underlying the Ehrlich hypothesis is not adapted to 

 the more modern views of the mechanism of immune reactions. The 

 combination of toxin and antitoxin shows numerous features not to 

 be explained by the simpler reactions of crystalloids. The same is 

 true of agglutination, precipitation, cytolysis, complement-fixation, 

 and anaphylaxis. 



Specificity of Immune Reactions. The antitoxin elaborated in 

 the response to injections of diphtheria toxin or to the presence of 

 the disease itself is a substance which reacts only with diphtheria 

 toxin. The agglutinins and precipitins produced by injection of 

 bacteria and of dissolved proteins act most powerfully upon the sub- 

 stances used for injection. In this case, however, these immune 

 bodies may also react less strongly with other closely related bac^ 

 teria or proteins. Cytolysins induced by the injection of certain 

 cells react strongly with those cells, but also less strongly with 

 closely related cells. This phenomenon of reaction with closely re- 

 lated bodies is spoken of as the group phenomenon and may be 

 exhibited also in connection with complement fixation and ana- 

 phylaxis. Even where purified proteins are employed the same 

 phenomenon may be observed. In spite of the group reaction, how- 

 ever, the immune substances are most highly specific for their spe- 

 cial antigens. Specificity has been employed for the detection of 

 particular proteins of animal species, of bacterial species, and it 

 has lent support to the Darwinian theory of species relationship and 

 evolution. Much thought and study has been given to the resem- 

 blance between immune substances and enzymes, but in no sense 

 can enzymes be said to have the same specific character as immune 

 bodies. There is no satisfactory explanation of specificity. Why 

 the injection of red blood-corpuscles of the sheep should induce the 

 formation of a hemolysin capable of dissolving the red cells of the 

 sheep but not of other animals, except in minor degree of those 

 closely related to the sheep, cannot be explained. As can readily be 

 understood, specificity involves the use of special antigens and the 

 formation of more or less specific immune substances. The wide 



