GENERAL PROPERTIES OF COMPLEMENTS 213 



same class with activities which are not yet known. It is 

 better to regard complement from the point of view of 

 chemical or physical combination, and define it to be that 

 labile substance of normal serum which is taken up by the 

 combination of an antigen and its anti-substance (immune- 

 body). The antigen of itself may not take up any appreci- 

 able amount of complement, e.g. in the case of haemolysis, 

 or it may take up a small amount as is seen in the direct 

 absorption of the complement by bacteria and tissue cells. 

 In this latter case the amount of absorption is, of course, 

 much increased by the addition of the homologous immune- 

 body. Various antigens combined with the homologous 

 immune-bodies are thus powerful complement absorbers, 

 and in some cases the complement content appears to be 

 practically exhausted after such treatment. The action of 

 complement as thus defined varies ; it may be haemolytic, 

 it may be bactericidal or bacteriolytic, and it may be 

 opsonic ; in one instance an agglutinative effect has been 

 described above. 



Complement is sometimes spoken of as a ferment-like 

 substance. There are certainly some similarities in the 

 physical properties of ferments and complements, but if 

 we study the mode of action, a wide difference is seen at 

 once. So far from complement not being used up in pro- 

 ducing its characteristic effect, we find that it enters in 

 definite proportion into firm union with the cell on which 

 it acts, and that in most cases there is no evidence of 

 subsequent dissociation. The maximum amount of 

 haemolysis by a given amount of complement thus 

 soon comes to an end and the process does not progress. 

 We have also shown that the amount of complement fixed 

 depends upon the amount of immune-body present, and 

 that in the case of hsemolytic sera many times the amount of 

 complement necessary for complete lysis may be fixed to the 

 receptors of the corpuscles. In some cases each additional 

 dose of immune-body leads to the union of additional com- 



