16 COLLECTED STUDIES IX IMMUNITY. 



not of simple constitution; but the experimental solution of this 

 problem was attended with great difficulties. 



The primary tests necessary to demonstrate the complex con- 

 stitution of a lysin are very readily made on a number of series. 

 They consist in this, that a serum which dissolves certain red blood- 

 cells at ordinary temperatures is mixed with these cells at and 

 allowed to act at this temperature for some time. For example, 

 goat serum is mixed with guinea-pig blood-cells, for which it is nor- 

 mally hsemolytic. The mixture is kept at and then centrifuged. 

 The clear fluid is mixed with an additional amount of blood-cells 

 and tested in the usual manner for its hsemolytic power. In this 

 way it was easily shown that through this procedure the serum had 

 lost part of its power, but that this was completely restored by 

 the addition of some of the same serum previously inactivated by 

 heat. According to our previous experience these experiments show 

 that this serum contains two substances: one, which we shall call 

 interbody, possessing two haptophore groups and analogous to the 

 immune body; the other, an addiment, which we shall hereafter term 

 complement. Further, they show that of these two bodies the blood- 

 cells combine preponderantly with the interbody. The decrease in 

 the power of the serum is thus explained by a lack of interbody, 

 and this is supplied by the addition of inactive serum. 



In experiments of this kind we have succeeded with the following 

 combinations : goat serum, sheep serum, calf serum, and dog serum, 

 with guinea-pig blood. 



Although the demonstration of the lack of interbody is 

 extremely simple, the counter-demonstration, that this interbody 

 has combined with the sedimented blood-cells, is extraordinarily 

 difficult; for in this demonstration a completely isolated comple- 

 ment is essential. The production of a complement to fit the specific 

 interbody obtained by heating the serum of our immunized goat 

 is extremely easy, for it is found in all normal goat serum and can 

 also be obtained from immune serum by means of elective absorp- 

 tion. 



It will be well to analyze the conditions governing this elective 

 absorption by means of which interbody and complement can be 

 separated. Complete separation will be possible when, under the 

 circumstances prevailing at the time, the affinity of the interbody 's 

 haptophore group for the blood-cells is greater than the affinity 

 of its haptophore group for the complement. A measure of the 



