BACTERIOLYSINS AND H^EMOLYSINS 65 



have no very great affinity for each other. At o C. 

 they may exist in serum side by side, and they 

 combine only at higher temperatures. 



The amount of immune body which combines 

 with the red cells may vary greatly, as the experi- 

 ments of Bordet and of Ehrlich clearly show. 

 Some red cells combine with only just enough 

 immune body to effect their solution. Others are 

 able to so saturate themselves with immune body 

 that they may have a hundred times the amount 

 necessary for their solution. 



On what the Specificity Depends. From the pre- 

 ceding it follows that the specific action of the 

 haemolytic sera, and, I may at once add, of the bac- 

 tericidal sera also, is due exclusively to the immune 

 body. This possesses a combining group which is 

 specific for the cells with which the animal was 

 treated; e.g., the combining group of an immune 

 body produced by treatment with rabbit blood 

 will fit only to a certain group in the blood cells of* 

 rabbits; an immune body produced by treatment 

 with chicken blood will fit only to parts of the red 

 cells of chickens; one produced by treating an ani- 

 mal with cholera bacilli will fit only to this species 

 of bacteria and combine only with the members of 

 it. Keeping to the well-known simile of Emil 

 Fischer, the relation is like that between lock and 

 key, each lock being fitted only by a particular 

 key. 



