178 THE PRINCIPLES OF IMMUNOLOGY 



The above protocol shows that anti-human serum is capable of 

 fixing complement in the presence of an amount of antigenic serum, 

 which is considerably less in the case of human antigenic serum than 

 in the case of monkey antigenic serum. Thus the group reaction is 

 indicated by the fact that the serum of a closely-related species is in 

 certain doses sufficient to produce fixation. Muir and Martin, also, 

 were able to demonstrate similar reactions in several different 

 animal groups. 



Relation of Complement-fixing Bodies to Other Immune 

 Bodies. The fact that the treatment of animals with a protein in 

 solution can lead to the development in the animal's serum of a capacity 

 both for precipitating the antigen and combining with the antigen to fix 

 complement suggests naturally that there may be some relationship 

 between the two phenomena. Gay and Moreschi independently were 

 able to show that precipitates formed by the action of a specific immune 

 serum can so bind complement as to prevent its action upon a hemolytic 

 system. The assumption is justified, therefore, that the two phenomena 

 are very closely related and may indicate that complement fixation 

 depends in part at least upon fixation of the complement by a precipitate. 

 The question naturally arises then whether or not there may be com- 

 plement fixation without precipitation or precipitation without com- 

 plement fixation. Furthermore, a fundamental problem is whether or 

 not the two activities of the antiserum depend upon two different 

 immune bodies in the serum or upon the double capacity of the same 

 immune body. Neisser and Sachs were able to show that complement 

 fixation occurred with very much smaller amounts of antigen than does 

 visible precipitation. As has been mentioned before, in reference to the 

 delicacy of the reaction, it was pointed out that fixation of complement 

 may occur with dilutions of 1-1,000,000,000, whereas visible precipita- 

 tion has never occurred in such marked dilution of antigenic or of 

 immune serum. Thus it can be concluded that the presence of a visible 

 precipitate is not necessary for the fixation of complement, a statement 

 amply corroborated by Muir and Martin. Wassermann and Bruck 

 found that by permitting bacterial extracts to stand for a considerable 

 time, the extracts were no longer precipitable in the presence of specific 

 precipitating immune sera, whereas fresh extracts show beautiful pre- 

 cipitation. Nevertheless, both new and old bacterial extracts were 

 found to fix complement in the presence of the specific immune serum. 

 Liefmann further showed that the action of heat may so alter the 

 antigenic protein as to lead to differences in complement fixation and 

 precipitation. He immunized rabbits with egg-white and found that 

 after heating the egg-white it could be so changed that it was no longer 

 precipitable by the immune serum but could still operate with the 

 immune serum in complement fixation. 



Felke and also Garbat have found that anti-typhoid vaccination in 

 man leads to the production of agglutinins, but to no or very slight 

 production of complement-fixing bodies. Felke found that in the course 

 of typhoid fever and during convalescence complement fixation could 



